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ELEMENTARY 
ELECTRICITY 

UP-TO-DATE 



A complete, practical guide for the beginner in the study of elec- 
tricity and electrical experiments, magnets, magnetism, in all its 
various aspects, circuits, and methods of designing, resistance, and 
the laws by which it is governed, the induction coil and principles 
controlling its action. The laws governing the flow of current are 
also clearly explained. :::::: 



SIDNEY AYLMER-SMALL 



PROFUSELY ILLUSTRATED 




CHICAGO 

FREDERICK J. DRAKE & CO. 

PUBLISHERS 



■% 



.$ 






$ 



Copyright 1909 

BY 

Frederick J. Drake & Co. 
Chicago. 



LIBRARY of CONGRESS 

Two Cooies Received 

MAY 15 1909 

COPY 3. 



PREFACE. 

Electricity presents a never ending field of research to 
the student who is inclined to delve among its mysteries. 
But a mere glance at the subject, with its apparently be- 
wildering array of facts, and problems, both old and new, 
has a tendency to confuse, and at times to discourage 
rather than to excite interest, therefore the only correct 
and profitable method to be pursued in its study is to be- 
gin at the bottom, so to speak, and gradually work one's 
way up. It is with this end in view that the following 
pages are presented. Starting out with an explanation of 
the nature of electricity, and electrical experiments of the 
most simple form, the book gradually, but surely, guides 
the student in his pursuit of knowledge of this most won- 
derful science. First comes static electricity, with a clear 
explanation of the principles governing the various meth- 
ods of its production. Condensers, what they are, and 
what they are for ; atmospheric electricity, and apparatus 
affected by it. Lightning arresters, and their auxiliary 
apparatus. Magnets, magnetic fields, magnetism, electro- 
magnetism, primary batteries, and how to make and care 
for them. Storage batteries of all types, together with a 
lengthened explanation of their construction, and the prin- 
ciples governing their action. Circuits, resistance and the 
laws by which it is controlled ; the induction coil, and its 
action; electrical measuring instruments, their construc- 
tion, and the principles governing their action. The flow 
of current, and the laws by which it is controlled, also re- 
ceive a large share of attention. 

Qhm's law is clearly explained, and numerous problems 

9 



10 PREFACE 

calculated according to its rulings, thus placing before the 
mind of the student the workings of this valuable law in 
a plain and simple manner. Rotary converters, and the 
principles of their action, together with the details of their 
construction and operation; transformers, what they are, 
and what they are for; transmission lines, and all of the 
various apparatus necessary in the transmission of current 
at a high voltage to locations at a distance from the cen- 
tral station, there to be transformed and made to do work, 
all are clearly and fully explained. 



LESSON i. 
Introduction. 

Exactly what electricity is we do not know, and indeed 
a practical man always dealing with the useful effects 
of electricity, applying these to the necessities and for 
the comfort of mankind, finds little time to wonder what 
electricity is. 

We will omit the almost unanswerable question: — 
"What is electricity?" — and proceed to the question. 

Question i. What is meant when a person talks about 
electricity ? 

Answer. There are certain effects which we believe 
to be due to electricity, so that when a person observes 
any one of these things, he at once declares that there is 
electricity present. 

Question 2. What are some of these effects? 

Answer. Lightning during a thunder storm. Light- 
ing gas with a spark from the finger tip. 

Question 3. Are these effects due to electricity? 

Answer. Yes, but by a kind which is different enough 
from other electricity to be called Static Electricity. 

Question 4. Why is it called static electricity? 

Answer. Because a quantity of this kind of electricity 
will remain on a body provided it is hung up by a dry 
silk thread. This electricity is at rest or stands still. 
Static means standing. 

Question 5. What other effects are due to electricity ? 
11 



12 ELEMENTARY ELECTRICITY 

Answer. Ringing of an electric bell. Operation of an 
electric motor. 

Question 6. Are these effects due to static electricity? 

Answer. No, the bell and motor are operated by 
Dynamic Electricity. 

Question 7. What is meant by dynamic electricity ? 

Answer. Electricity in motion, because the word 
dynamic means force, giving the idea of motion. 

Question 8. What is meant by a current of electricity? 

Answer. Dynamic electricity is usually referred to as 
a current of electricity. 

Question 9. Are there any other effects said to be due 
to electricity? 

Answer. Yes ; the working of a wireless telegraphy 
instrument. 

Question 10. What kind of electricity operates this in- 
strument ? 

Answer. Electrical waves. 

Question 11. Are there still other kinds of electricity? 

Answer. Practically no. All the effects we observe 
may be explained as being caused by either static elec- 
tricity, a current of -electricity or electrical waves. 

Question 12. What is static electricity? 

Answer. It is electricity at rest. 

Question 13. What is current electricity? 

Answer. It is electricity in motion along a conductor. 

Question 14. What are electrical waves? 

Answer. They are electricity moving through the air, 
no conductor being required. 

Question 15. Does the word electricity in the last 
three answers mean the same thing in each? 

Answer. Yes; in each case it is electricity, either at 



INTRODUCTION 13 

rest, moving along a wire, or moving through the air 
without a wire. 

Question 16. Give an example of static electricity. 

Answer. If a brass ball be suspended by a silk thread 
and touched to one knob of an electrical machine (see 
Lesson 6) and then removed, it will be covered with a 
charge (see Lesson 2) of electricity. This electricity 
will be at rest and so is called a charge of static elec- 
tricity. 

Question 17. Give an example of a current of elec- 
tricity. 

Answer. If quantities of electricity are supplied at the 
end of a metal wire, they will quickly flow to the other 
end. Here the electricity is in motion and a current of 
electricity is flowing. 

Question 18. Give an example of electrical waves. 

Answer. If electricity is forced by a high pressure as 
in an electric machine or in an induction coil (see Lesson 
2j) to jump a spark across an air space, while doing so 
it will send out in every direction a series of electrical 
waves which will travel long distances without the aid 
of wires. 

Question 19. What are some of the common effects 
of current electricity? 

Answer. Heat, light, magnetism, metal plating and 
refining, and medical effects. 

Question 20. Explain about the heat effect. 

Answer. If large quantities of electricity are forced 
through a conductor in a short time the wire is heated. 
The poorer the conductor the more heat is produced. 

Question 21. Is there always some heat produced? 

Answer. Yes. Electricity cannot flow through a con- 
ductor without producing some heat. 



14 ELEMENTARY ELECTRICITY 

Question 22. Are the wires in a building heated, while 
carrying electricity? 

Answer. Yes ; but the wire is large compared with the 
current carried; the material is copper, a good conduc- 
tor, so that the heating is too small to be detected by feel- 
ing the wire. 

Question 23. Does electricity produce light? 

Answer. Yes. The heat produced in a very poor con- 
ductor by the current may be so great as to burn the con- 
ductor and the flame gives light; or it may make the 
conductor white or yellow hot, thus giving light. 

The arc lamp gives light from flame and white hot car- 
bon, while the incandescent lamp has no flame only the 
yellow hot carbon. 

Question 24. Does electricity produce magnetism? 

Answer. Yes. Electricity in motion will always affect 
the needle of a compass, usually pulling it aside from the 
north and south line, and keeping it out. This is de- 
scribed as "deflecting the needle ,, and whenever the 
"magnetic needle" is spoken of we mean a magnet of 
small weight and fairly long, pivoted so as to move freely 
in a horizontal direction. We speak of this effect as 
electro-magnetism. 

Question 25. How does electricity plate metal? 

Answer. Electricity in passing through solutions of 
chemicals takes the metal out of the solution and turns it 
into the solid form, thus making a layer of metal on the 
object placed in the solution. (See Lesson 17.) 

Question 26. How does electricity refine metals ? 

Answer. If a lot of metals and other chemicals are in 
a solution, by passing electricity through the solution the 
metals will be solidified and may be removed while the 
other chemicals remain dissolved in the solution* 



INTRODUCTION 15 

Question 27. What are the medical effects of elec- 
tricity ? 

Answer. They are not well understood; but the pas- 
sage of current through the body seems to have a curative 
effect on some diseases. 



Introduction to Static Electricity. 

Static electricity is of importance to the railroad man 
in many ways. 

In power houses and machine shops the belts often 
produce static electricity so that a person going near or 
under them will receive a rather unpleasant shock. 

A locomotive blowing off steam through the safety 
valve becomes electrified but the charge is much too 
small to give any one a shock. 

Lightning is static electricity and consists of such large 
charges that electrical machinery is usually badly dam- 
aged if lightning passes through it. 

The telegraph, telephone and signal circuits, the power 
lines, and all buildings into which wires enter must be 
protected by lightning arresters. 

Motor cars and electrical locomotives must be equipped 
with lightning arresters to protect their wiring, appa- 
ratus and motors. 

A great many of the cables distributing electricity, 
especially at large railroad terminals, or along the right 
of way in the city limits are carried in conduits under- 
ground. To protect them from moisture they are covered 
with lead. This lead sheathing often collects electricity 
and heavy static discharges take place. 

The discharge may injure instruments and apparatus 
connected to the cables or may even injure the power 
house or line men while handling the cables or switches 
attached to them. 

A proper arrangement of lightning arresters and static 
dischargers will prevent this. 

is 



Introduction to static electricity 



17 



It will be seen that a thorough understanding of static 
electricity is a good thing for a railroad man. 

There are three pieces of apparatus which are easily 
made, and the use of whicih will help one to readily under- 
stand the action of static electricity. 

They are the Electrophorous, to produce static elec- 
tricity ; the Leyden Jar, to store electric charges in ; arid 
the Electroscope, to detect the presence of an electric 
charge and its polarity. 



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Fig. 1. Construction of an Electrophorous. 



The Electrophorous. 

Melt a mixture of two-thirds rosin and one-third gum 
shellac or some common red sealing wax,* by setting the 
dish on hot water. This is to avoid the danger of its 
catching fire. 

* Sealing wax is a mixture of rosin or other resins with 
yermilion or some other powdered color. 



18 Elementary electricity 

Poiif the rrlelted stuff into a large tin pie plate and 
allow to cool into a solid cake. Slow cooling will pre- 
vent cracks, but they really do no harm. 

Get a tin pie plate or tin layer cake pan a little smaller 
in diameter than the resin* cake. Make a wood cone. 
Screw a large flat head wood screw into the thick end 
of cone, and then solder it to the tin plate. Fasten it to 
the inside of bottom of plate. 

Whittle the small end of the cone to a driving fit to the 
neck of a small glass bottle. Press the bottle on gently 
but firmly. 

We now have a glass-handled tin dish to rest on the 
resin cake, touching it all over, but the two tin plates not* 
touching anywhere. 

A piece of flannel or woolen cloth is needed for a rub- 
ber. 

Leyden Jar. 

A large plain beaker should be purchased from a chem- 
ists' or druggists' supply house. The "plain" means that 
there is no lip on the edge. It should be large enough 
to hold a quart and a half of water. It will be a very 
thin glass and must be handled carefully to avoid break- 
ing. 

Cut tinfoil to fit the inside and outside of the bottom 
and dry the beaker over a stove or radiator. While dry 
and warm paste the tinfoil on. Warm it again and paste 



* Resin means any gum that flows from a tree as a sticky 
liquid and hardens on contact with air. Gum arabic and gum 
shellac are resins. Rosin is a resin from a pine tree. It is 
left in the bottom of the stills when the spirits of turpentine 
are boiled off. 



INTRODUCTION TO STATIC ELECTRICITY 



19 



tinfoil over the outside up to about two-thirds its height. 
Let the side foil lap over the bottom foil. 

Cut a piece to fit the inside and roll it around a pencil. 
Coat the inside with paste to about two-thirds its 
height with paste or mucilage and stick one end of the 
tinfoil down. Unwind the foil from the pencil and press 
it down on paste with ringers or another pencil. 

If the side and bottom foils do not connect, paste a 
strip across the seam. 

Take a thin board larger than the mouth of the beaker 
and dry over a stove and shellac varnish it while hot. 




Fig. 2. Leyden Jar. 



Warm the beaker and while warm and dry shellac 
all the glass not covered with foil. 

Drill a small hole in the board and insert a short metal 
rod. Fit to its lower end a piece of chain long enough to 
touch the foil on the bottom of beaker, when the board 
rests on its top. Fit the upper end with a metal ball. 
Any size, solid or hollow makes no difference. 



20 ELEMENTARY ELECTRICITY 

Put a circle of shellac varnish on the under side of 
board and lay board on beaker so that chain touches bot- 
tom. 

When shellac dries the board will stick and keep inside 
'of jar dry. 

Discharger for Leydex Jar. 

Bend a piece of \vire like this-v- and stick the doubled 

part in a cork, put cork in a glass bottle and the result is 
as good as Fig. 3. 




Fig. 3. Discharger or Discharging Tongs for Leyden Jar. 

Electroscope. 

A wide-mouthed fruit jar is taken and a cork or wood 
stopper made to a loose fit. 

Drill a hole through the stopper and insert a glass tube 
about four inches long projecting equally on both sides. 

Take a small metal rod or heavy wire which will go 
through the glass tube and hammer one end flat for about 
half an inch from the end. The thinner and flatter it is 
made the better. 



INTRODUCTION TO STATIC ELECTRICITY 



21 



Insert wire in tube so that the flattened end is about 
one-third the way down the jar when cork is in place. 
Cut off surplus wire at other end, leaving about half an 
inch projecting from the glass tube. 

Fix a small metal ball or plate on end of rod and let it 
drop down and rest on the end of glass tube. 




Mi 

Fig. 4. Gold Leaf Electroscope. 



Hold ball up against tube and turn it upside down, 
pouring shellac varnish into the tube till it is filled. Give 
the cork a coat of shellac* also. 

When dry the metal rod will be cemented in the tube 
and the tube into the cork. 

Give outside of glass rod, the cork, the neck of bottle 



* A pure shellac varnish is meant, made by dissolving or 
"cutting," as it is called, flakes of gum shellac in alcohol. Wood' 
or grain alcohol are equally good. Orange or brown shellac 
refer to the color of gum. Either will do. 



22 ELEMENTARY ELECTRICITY 

inside and out two coats of shellac, allowing time for 
perfect drying between coats. 

The cork will now be a good tight fit. 

Scrape any shellac off the flattened end of the rod and 
paste on two strips of gold leaf, silver leaf, dutch metal 
or aluminum foil; as broad as the flat end itself is wide 
and long enough so as to just not hit the sides of the jar 
if they should stand out straight apart. 

Cover the bottom of the jar an inch deep with fresh 
dry calcium chloride. Buy it the day you are going to 
put it in jar. 

Give a last coat of shellac to the outside edge of the 
cork and putting it in jar press down firmly, seeing that 
the rod with its foil leaves hangs straight. 



LESSON 2. 
Static Electricity. 

Question I. What is static electricity? 

Answer. I think it is a very small quantity of elec- 
tricity at a very great pressure. 

Question 2. Why do you think the quantity is small? 

Answer. Because we cannot get any steady power out 
of static electricity. 

Question 3. Lightning often does considerable dam- 
age. How do you account for that ? 

Answer. The damage done by lightning is like a blow 
or an explosion, and seems to be the result of great pres- 
sure. 

Question 4. A -static machine gives steady power, i. e., 
a steady stream of sparks. 

Answer. No, it doesn't. The sparks follow each other 
rapidly but are really intermittent. There is more time 
between sparks than the time of a single spark. 

Question 5. Why do fou think it has such a great 
pressure ? 

Answer. Because it will jump across distances which 
current electricity will not. 

In all that we see of static electricity we are constantly 
reminded of the fact that it possesses a power to escape 
which enables it to cross a considerable space of one of 
the best insulators known — dry air. This must mean 
that static electricity is at a high pressure. 

23 



2-i ELEMENTARY ELECTRICITY 

Question 6. Make this clearer. 

Answer. A switch in an electric circuit can be almost 
closed and yet no current will pass till the switch is 
closed. Static electricity would have jumped across 
when the opening between the switch blades became 
small. 

Question 7. Mention some easy ways of producing 
static electricity. 

Answer. I. If you will rub a fountain pen on your 
coat sleeve it will attract small pieces of paper. 

2. A piece of glass rubbed with a silk necktie will do 
the same. 

3. A spark will be produced by shuffling your feet 
along the carpet and then touching a gas jet. This is in 
miniature the same effect as lightning during a thunder 
shower. 

Question 8. Are there still other ways to produce 
static electricity? 

Answer. Yes. 1. Friction between two different 
substances will always produce electricity unless the 
dampness is excessive. A leather belt on a rotating pul- 
ley will become charged and give you a severe shock if 
you walk under it. 

2. Percussion. A violent blow struck by one sub- 
stance on another produces positive and negative charges. 

3. Breakage. Tear a playing or a visiting card, or 
even a linen paper shipping tag suddenly across, and you 
will charge the two pieces. Crunching a lump of sugar 
quickly between the teeth does the same. Split a sheet of 
mica with a sudden motion and you will also charge the 
pieces. 

Any of these tests done in a very dark room will show 



STATIC ELECTRICITY 



25 



faint sparks. The hard rubber slides of the plate holder 
of a camera, if pulled out suddenly on a dry day may 
make such a spark that you can hear the crackling noise 
above the sound made by the slide itself. 

4. Solidification. When melted chocolate is poured 
into moulds, upon cooling and hardening it becomes 
electrified. 

5. Combustion. The burning of a joss stick to keep 
away mosquitos causes it to become electrified. 




Fig. 5. Attraction due to Static Electricity. 



6. Evaporation. The evaporation of a liquid from its 
surface, when that surface is roughened by waves, pro- 
duces electrification. This is one of the ways that the 
atmosphere is kept charged relatively to the earth. In 
fair weather it is always positive to the earth and during 
rains and storms it is sometimes negative. 

Question 9. Mention some other ways of showing 
static electricity. 

Answer. Briskly rub a sheet of paper which is lying 
on a polished desk, with a rubber eraser, or even the 
hand. If the room is cool and dry the sheet will stick 
to the table. 



26 ELEMENTARY ELECTRICITY 

If two sheets are laid down together and rubbed and 
both pulled away together; then when these are pulled 
apart they will forcibly repel each other. 

Lay a glass rod on a table, one end of which extends 
over the edge a few inches. Attach to this a silk thread,* 
fastened to the lower end of which is a small ball of pithf 
from an elder or corn stalk. 

Rub a second glass rod with a silk handkerchief and 
bring it near the pith ball. It will be strongly attracted, 
but almost immediately repelled, and it will not approach 
the glass rod again. 

Now rub a stick of sealing wax or a piece of wood 
highly polished with shellac varnish, with a woolen rag 
or piece of flannel and bring this near the pith ball. It 
will be attracted and then repelled. We may repeat 
this attraction and repulsion as often as we please. 

Question io. I do not understand the repulsion. What 
causes it? 

Answer. It is explained by saying that there are two 
kinds of static electricity or at least two states of it. 

Question n. Why does it seem that there are two 
kinds of static electricity? 

Answer. The fact that under one set of circumstances 
an electrified body will be drawn to another body, and at 
other times will be repulsed by the same body, plainly 
indicates that there are two electrical states, one of attrac- 
tion and the other of repulsion. 

Question 12. Explain this further. 



*A single thread drawn from a piece of embroidery silk is 
best because it is very light and flexible. 

t Any pith will do, and it may be purchased at a drug store. 



STATIC ELECTRICITY 27 

Answer. The glass rod in Ag* attracted the pith ball 
and then repelled it. Since it acted differently towards 
the same charge of static electricity, there must have been 
a charge on the pith ball which had two conditions, one 
where it attracted and one where it repelled. 

Question 13. How can these different states be pro- 
duced or induced, as it is called? 

Answer. Different electrical conditions are produced 
by different treatment of the bodies electrified. The glass 
rod rubbed by a silk handkerchief induced a condition 
which is unlike that induced by the flannel's friction on 
the sealing wax or shellac of the varnished wood. 

Question 14. But this does not explain about the re- 
pulsion or the cause of it. 

Answer. With the knowledge gained in this experi- 
ment and what we already know, we can find an explana- 
tion. 

Lay two glass rods over the edge of the table and as 
far apart as possible and attach pith balls to each by 
silk threads. 

To one present a glass rod rubbed with silk. It is first 
attracted and then repelled. 

To the other present a stick of sealing wax that has 
been rubbed with flannel. It will be attracted and then 
repelled. 

Take away the glass rod, the stick of wax, the silk 
and flannel to a distance. Pick up one of the glass rods 
and slowly bring its pith ball up towards the other. They 
will be attracted. 



♦References to Answers in same Lesson will be made in this 
way. 



2^ ELEMENTARY ELECTRICITY 

Handle the pith balls with the fingers a few seconds to 
dissipate their charges. Replace them as before. 

Rub the glass rod with silk and present to each pith 
ball. They will be attracted and repelled. 

Xow as before bring the pith balls together and they 
will repel each other. 

Discharge the pith balls and repeat this last part, using 
the sealing wax rubbed with flannel. 

The balls are attracted and repelled, and when brought 
near together they repel each other. 

We already know from Ag that an unelectrified body 
is always attracted to an electric charge before being re- 
pelled. 

From these demonstrated facts we deduce the follow- 
ing: 

Similar electric charges repel each other. Dissimilar 
charges attract each other. Either electrical condition 
may show attraction for an unelectrified body. 

In short: — 

Like charges repel. 

Unlike charges attract. 

Charges attract neutral bodies. 

Question 15. This explains why the two pith balls 
were first attracted when one was charged by glass rod 
and the other by sealing wax. 

It explains why they repelled each other when both 
were charged by glass rods. 

Explain why a neutral or unelectrified body is attracted 
and then repelled. 

Answer. We have come to the conclusion that all 
bodies contain equal amounts of the two kinds of static 
electric:: 



STATIC ELECTRICITY 29 

When a body rests undisturbed the opposite qualities 
of the parts neutralize each other and no outside effect 
is observed; indeed one is tempted to say that there is 
no electricity in the substance. 

In a similar manner drinking a glassful of a solution 
of caustic soda or of diluted muriatic acid would corrode 
the lining of your stomach, and perhaps cause death. 

Mixing the two would, if the chemicals were clean and 
pure, and present in the right quantities, produce a large 
glassful of a solution of table salt in water. I would 
have no fear in drinking this, except for the excessive 
thirst sure to follow such a salty beverage. 

So the question is answered in this way : — 



<£ 



3 




Fig. 6. State of a body B, having a charge induced in it due to the 
action of C. 



An unelectrified pith ball is really only an uncharged 
one, for the two kinds of electricity are in it in equal 
quantities and neutralize each other. 

When the glass rod charged with glass electricity was 
brought near the pith ball, the glass electricity of the 
glass rod repelled the glass electricity of the pith ball to 
the far side, and attracted the other kind (resinous) to 
the near side. See Fig. 6. 

The attraction being nearer than the repulsion the 
whole pith ball with its separated electricities is pulled 
toward the glass rod. 

When the pith ball gets very near the glass rod the 



30 ELEMENTARY ELECTRICITY 

attraction of the glass electricity on the rod actually pulls 
the other kind (resinous) off the pith ball, leaving only 
the glass electricity. The glass rod now repels the pith 
ball even more strongly than it attracted it before. 

When the stick of sealing wax was presented to a pith 
ball the same thing is done by the resinous electricity of 
the stick, it repels the resinous kind and attracts the glass 
kind. The same things occur and the ball is first at- 
tracted, then repelled. 

Question 16. Are these names, glass and resinous, 
actually used? 

Answer. Vitreous and resinous have been used, but 
these names are out of date and they are now known as 
positive and negative electricities. 

The names are usually abbreviated by using the sign + 
for the positive, and — for the negative. 

Question i~. Then an electric charge can induce a 
charge in another body, that is, cause its electricities to 
separate, without actually touching it ? 

Answer. Yes. as is shown in this experiment. 

A sphere of metal, or wood covered with tinfoil is 
mounted on an insulating stand — a wooden stand with a 
glass rod for its support. A second similar stand has a 
horizontal cylinder of conducting material, or wood cov- 
ered with foil, hanging from which are double threads of 
silk ; and to these two or three inches below the cylinder 
are fastened little pith balls. See Fig. 30, on page ~&. 

These double threads, four or five in number, are dis- 
tributed along the cylinder at regular intervals. 

The little cylinder, say an inch in diameter and six 
inches long, is now insulated from the ground. 

If the sphere is now charged and brought near one 
end of the cylinder, each pair of pith balls will show re- 



STATIC ELECTRICITY 31 

pulsion and remain standing apart. Those at the two 
ends of the cylinder will show the greater repulsion and 
remain further apart than the pairs near its center. 

If we now electrify a rubber comb or glass rod by rub- 
bing it, we will see that, on approaching the pith balls, it 
will attract those at one end and repel those at the other ; 
thus showing the ends of the cylinder to be oppositely 
electrified. 

This proves conclusively that the approach of the 
charged sphere separated the two electrical conditions on 
the cylinders, attracting the opposite kind and repelling 
the same kind. 

It also gives another proof that "Like charges repel," 
because each of the pith balls in a pair had the same kind 
of electricity in them, and repelled each other. 

Question 18. Does the same charge always induce the 
same quantity of electricity? 

Answer. Yes, as is shown in the following experi- 
ment 

As shown in Fig. 7, insulate a tin can by standing on a 
glass tumbler. Connect it to an electroscope.* 

Charge a metal ball and lower into the can by a silk 
string, being careful not to let it touch the sides. 

As the ball is lowered a charge is induced in the tin 
pail and the gold leaves of the electroscope diverge, show- 
ing it. 

When the ball is well down into the pail the leaves 
will diverge no further, even if the ball is touched to the 
bottom. 

This proves that the ball induced an equal and opposite 



* Turn back and read description of Electroscope. 



32 



ELEMENTARY ELECTRICITY 



charge in the pail because touching the pail with the ball 
added no more electricity. 

Furthermore, after the ball has touched the pail pull it 
out and it will be found perfectly neutral. 

It was neutralized by the equal and opposite charge 
which it induced. 




Fig. 7. 



Showing that the induced charge is equal and opposite to the 
inducing charge. 



The electroscope was operated by the equal and like 
charge left free by the induction of the opposite charge. 

Question 19. What is the usual trouble when experi- 
ments such as have been described won't work ? 



STATIC ELECTRICITY 33 

Answer. The things used are not perfectly dry or the 
air is too moist. 

Question 20. What other things might cause trouble ? 

Answer. If the table used were set over a hot-air 
register, the current of hot air will carry away the elec- 
trical charges, perhaps as fast as produced. 



LESSON 3. 
Static Electricity (Continued). 

Question 1 . Can you induce a + charge without at the 
same time inducing an equal — charge ? 

Answer. No. 

Question 2. But when you rubbed the fountain pen, 
or the glass, or the carpet, as in Ay, Lesson 2, only the 
pen, glass or body seemed to have charges. 

Answer. Careful investigation will prove that the coat 
sleeve, necktie and the carpet under foot contain electrical 
charges as well as the pen, glass, and your body. 

Question 3. Why careful investigation? 

Answer. Because it is very easy to be deceived by ap- 
pearances in any electrical test. Care must be taken that 
real effects are observed and to properly understand what 
we see. 

Question 4. What test is usually applied to the coat 
sleeve, necktie and carpet to see if they are electrified, 
i. e., if they contain a charge of static electricity? 

Answer. A trial is made to see if they will attract pith 
balls or affect an electroscope. 

Question 5. But they won't and therefore there is no 
charge on them. 

Answer. No, not at the time they were tested, but 
while being rubbed or rubbing they were electrified. 

Question 6. Where did the charge go ? 

Answer. To the ground. You see the pen and glass 
rod were insulators and insulated (Lesson 2) so that 

34 



STATIC ELECTRICITY 35 

the electrification produced by the friction remained on 
them, but the coat sleeve and silk necktie were not such 
good insulators and were grounded, so that the charge 
flowed away. 

Question 7. But the glass rod and silk necktie were 
both held in the hands. How could one be insulated and 
the other not ? 

Answer. The glass rod being fairly long its lower 
part acts as an insulator for the upper part. The hand, 
which is a good conductor, makes a contact of small area 
with the lower end of the glass rod. Hence the charge 
on the other end, say three inches away, is insulated. 

The necktie makes a contact of large area with the 
hand and the charge being on the other side of the silk, 
say a few thousandths of an inch away, is not properly 
insulated and leaks away. 

Question 8. But the charge on your body when you 
drew a spark from the gas jet, did not flow away. 

Answer. No; because the carpet was the better 
grounded of the two and, moreover, the charge in the car- 
pet, while being rubbed, was actually there and repelled a 
charge into the body and held it there. 

After the spark to the gas jet the charge in the carpet 
flowed away to the ground, and any excess charge on the 
body also flowed away to ground. 

Question 9. How can you prove that the coat sleeve 
and necktie had charges while being rubbed? 

Answer. By mounting a piece of woolen cloth to rep- 
resent the coat sleeve on a glass rod and using this rod 
as a handle, rub the cloth on the fountain pen. 

Both the cloth and the pen will become charged, as 
the charge on the cloth can not flow away, it remains, can 
be tested and proved to exist. 



36 



ELEMENTARY ELECTRICITY 



The same idea may be carried out with a scrap of silk 
and a glass rod. 

Question 10. How do you test to prove a charge ex- 
ists ? 

Answer. By using an electroscope. 

Question 1 1. Describe an electroscope. 

Answer. As shown in Fig. 4 and in Fig. 8, it consists 
of a glass jar sealed up, practically moisture proof, with 
two leaves of metal foil hanging from a metal rod in the 
jar, the other end of the metal rod in the air terminates 
in a metal ball. 




Fig. 8. Charging an Electroscope by Influence. 



Two strips of foil are often pasted on the sides of the 
jar to discharge the foil leaves if they swing out too far. 

Question 12. How do you prepare an electroscope for 
a test? 

Answer. Rub a glass rod with a piece of silk and hold 
rod near the ball of the electroscope, but do not touch it. 
See Fig. 8. The leaves will diverge and remain apart. 

While keeping the glass rod near the ball touch the 
ball with the other hand and remove hand. The leaves 



STATIC ELECTRICITY 5? 

fall together. Remove the glass rod to a distance and 
the leaves will diverge again. 

Question 13. Why does the electroscope act this way 
while being charged? 

Answer. When the charge of + electricity on the 
glass rod is held near the neutral electroscope it separates 
the electricities and attracts the — to the ball at the top 
and repels all the + to the leaves at the bottom of the 
metal rod. 

The leaves being now both positively charged repel 
each other and diverge. 

When the ball is touched with the finger the — elec- 
tricity is held "bound" by the glass rod. Any + which 
would run in from the earth through the hand, is kept 
out by the + of the glass rod, while more — is attracted 
from the earth. This charge combines with the + in the 
leaves and neutralizes so -that the leaves are no longer 
charged. They therefore stop repelling each other and 
fall together. 

The — in the ball is still held "bound" by the + of the 
glass rod and can do nothing. 

The finger being removed, and then the glass rod being 
taken away, the — charge which was in the ball spreads 
all over the ball, rod and leaves. 

The leaves being now each charged with — they repel 
each other and diverge. 

Notice that the final charge of the electroscope is op- 
posite to the charge of the charging body. 

Question 14. How do you test for a charge? 

Answer. Bring the body on which a charge is sus- 
pected near the ball and if the leaves diverge further or 
fall together there is a charge on the body ; if they are 
not affected there is no charge on the body. 



38 ELEMENTARY ELECTRICITY 

Question 15. How do you determine the kind of elec- 
tricity on the body ? 

Answer. If the leaves diverge further when a body is 
brought near the ball its charge is the same kind as is in 
the electroscope, if they come together it is charged with 
the opposite kind. 

Question 16. What electricity is the electroscope usual- 
ly charged with? 

Answer. Usually a glass rod carrying a + charge is 
used, which induces a — charge in the electroscope. 

Then a further divergence of the leaves means a — and 
a collapse of the leaves means a + charge on the tested 
body. 

Question 17. What is the general rule for determin- 
ing the kind of electricity with an electroscope? 

Answer. Collapse of leaves means same kind of 
electricity as was used to charge the electroscope. Di- 
vergence of leaves means different kind. 

Question 18. How can you charge any body so as to 
have only one kind of electricity in it? 

Answer. By inducing a separation of the electricities 
by means of a charged body and then drawing off the 
"free" charge by touching the body with the finger. 

Question 19. What does bojund and free charges 
mean? 

Answer. A bound charge means a charge which is at- 
tracted and held by a charge of the opposite kind. 

Touching a body with a bound charge has no effect 
upon it, because it is not free to neutralize with any elec- 
tricity that might flow in, nor can it flow away, since it is 
held by the other charge. 

A free charge is a charge which is under no influence 
Or under the influence of a charge of the same kind. 



STATIC ELECTRICITY 39 

When a body is touched the free charge is either neu- 
tralized by the inflowing charge or flows away itself to 
the earth. Probably both of these actions take place. 

Question 20. What is the best way to get a fairly large 
charge of electricity ? 

Answer. By the Electrophorous. 

Question 21. Describe the Electrophorous. 

Answer. As shown in Figs. 1 and 9 it consists of a 
plate of resin resting on a metal plate, and a second metal 
plate with an insulated handle, rests on the resin. 




Fig. 9. Using an electrophorous. 

Question 22. How is the Electrophorous used? 

Answer. The upper plate or cover is removed and the 
cake of resin is rubbed or beaten with a piece of warm 
dry flannel or cat's skin. 

The cover is then taken up by the glass handle and 
placed lightly on the cake. With thumb and forefinger 
touch the metal plate containing the resin cake and the 
metal cover at the same time. 



40 



ELEMENTARY ELECTRICITY 



Lift the cover off by the handle and it will be charged. 
Question 23. How can you prove it? 
Answer. By drawing a spark from it by the ringer. 
Question 24. Do you rub the resin cake each time 
you want a spark? 



AG 



<n 



DA 



IB 



Fig. 10. The Electrical State of an Electrophorous vhen cover is on, 
and then after being touched and cover lifted. 



Answer. No. The cover may be replaced, touched as 
before, removed, and a spark drawn, over and over 
again. If left after being used the charge on the cake 
will leak away and rubbing will be necessary. 

Question 25. Explain how the Electrophorous works. 

Answer. When the resinous cake is first beaten its 
surface is — ly electrified and the + driven into the metal 
pan. See Fig. 10. When the metal cover is placed light- 
ly on the cake it does not touch at all points and is 
really an insulated conductor. 

The — charge on the resin cake acts by influence on 
the cover and induces a -j- charge on its under side and 
repels the — charge to the upper side. 



STATIC ELECTRICITY 41 

When the cover and the bottom pan are touched by the 
finger, several things happen. From the earth the resin 
cake attracts more + charge to the lower side of the 
cover and repels the — on the upper side of cover out to 
the earth through the finger. This gives the cover a 
larger + charge than before. This -j- charge is bound 
and cannot escape. 

The connection of the bottom pan to earth allows its 
-J- charge to be increased, which in turn helps to "bind" 
the — charge on the resin. 

If the cover is now lifted by the insulated handle the + 
charge on it spreads over it and it is charged ready to 
give a spark. 

Question 26. Will the Electrophorous work if only 
the cover is touched with the finger? 

Answer. Yes, but the spark is stronger when both 
cover and pan are touched. 

Question 27. Is the original charge used up in draw- 
ing several sparks in succession? 

Answer. No, because the electricity which gives the 
3park is really drawn from the earth each time. 

Question 28. You have obtained energy without any 
cost, have you not? This is contrary to nature's laws. 

Answer. No. The cost of the spark is the muscular 
effort of the first rubbing, and the subsequent touching 
and liftings. 

Question 29. How can a larger charge be obtained? 

Answer. By accumulating the small charges given by 
the electrophorous. 

Question 30. What is used to accumulate charges? 

Answer. A Leyden Jar. 

Questional. What is a Leyden Jar? 

Answer. It is a glass jar lined inside and outside with 



42 ELEMENTARY ELECTRICITY 

tinfoil with a conductor passing through an insulated 
cork touching the inside foil. See Fig. 2. 

Question 32. How is the Leyden jar used? 

Answer. To get the best results connect the outer coat- 
ing of tinfoil with a wire or chain to a gas or water 
pipe, thus giving a good connection with the earth. 

When the cover of the electrophorous is lifted, present 
it to the knob of the jar and let the spark jump to it. 
Repeat this a dozen times, and the jar will be charged. 

Question 33. How do you show this charge? 

Answer. Connect the ball with the outer coating using 
the discharger (Fig. 3) and a heavy spark will be ob- 
tained. 

Question 34. Will it be twelve times as large as the 
electrophorous spark? 

Answer. No. It will be much larger than the electro- 
phorous spark, but not twelve times as large, for there 
are leaks and other losses which will reduce its size. 

Question 35. Explain the main leak. 

Answer. Glass is a hygroscopic substance. This 
means that moisture collects on its surface very easily. 
A china tea cup will have a dry surface in a room, while 
a glass tumbler alongside of it will have a very damp 
surface. 

The charge leaks from one coating of tin foil to the 
other over this film of moisture. 

The shellac varnish on the glass is to prevent this 
film of moisture forming. It does this very well. 

Question 36. What is another cause of leakage? 

Answer. There is the leakage from the coatings to 
the air, because particles of air become electrified and 
are then repelled. 

Question 37. Why does not all the charge leak away? 



STATIC ELECTRICITY 43 

Answer. In a poorly made jar it will ; but with well 
shellaced glass, used in a dry atmosphere the leakage is 
slow, and the binding influence of the inner and outer 
charges on each other retains the charge. 

Question 38. Is there any other leakage? 

Answer. Yes there is leakage through the glass. 
While glass is principally sand and soda yet there are 
other ingredients added according to the grade of glass. 
The cheaper kinds have accidental ingredients, really im- 
purities, which lessen its value for electrical purposes. A 
general rule is that the more expensive the glass the bet- 
ter for electrical work. 

A ley den jar to approach nearest to electrical perfec- 
tion should be of glass containing no lead or other con- 
ducting material. 

For simple experimenting a cheap glass such as bottles 
or fruit jars are made of will do. 

In such cases it is absolutely necessary to coat the 
glass with shellac varnish before putting on the tin foil. 

Question 39. How can a larger spark be obtained? 

Answer. By accumulating more electrophorous 
sparks. 

Question 40. Is there any limit to the size ? 

Answer. Yes; in time the pressure of the charge to 
escape will be so great that the charge will either leak 
as fast as it is added to or the pressure will flash a spark 
over the edge of the glass. 

Question 41. Will a larger leyden jar give a larger 
spark ? 

Answer. Yes, a jar large enough to have twice the 
amount of tin foil put on will give twice as large a spark. 

Question 42. What do you mean by "twice as large 
a spark?" 



44 ELEMENTARY ELECTRICITY 

Answer. Either twice as long a spark, the same 
length but twice as thick or some combination like this. 
Twice as long and twice as thick, would be a spark four 
times as large. 




Fig. 11. A Leyden Jar with Removable Coatings. 

Question 43. Can several leyden jars be used at once? 

Answer. Yes, stand them all on a sheet of metal, and 
connect all the knobs together with a piece of wire. This 
is now the same as one big jar. Fig. 27, page 75. 

Question 44. How does a leyden jar work? 

Answer. The theory of the leyden jar is best explained 
through the aid of an experiment,, which requires a pe- 
culiar form of jar, but which is easily constructed. 



STATIC ELECTRICITY 45 

Take a large glass tumbler, and varnish inside and out 
with shellac. Arrange the inner and outer coatings so 
as to be removable. Fig. n shows one where the coat- 
ings have been made thick and stiff to facilitate their re- 
moval. 

A jar may be arranged more simply by attaching a ball 
of tin foil refuse to the lower end of the rod so that it 
will rest on the bottom of the jar and form the inner 
coating. 

For the outer coating thin sheet metal or foil may be 
wrapped around the jar and tied with thread. 

Place a board on several glass tumblers thus making 
an insulated stand. Place jar on stand. 

Charge the jar. Lift out the rod and inner coating — 
being careful not to come within sparking distance of 
the outer coating — and place on board. Now lift the 
jar from the outer coating and place on board. 

In each of these contacts a very slight shock will be re- 
ceived by the hand. 

A test of the two coatings will show that they are 
neutral, as is natural since any charge on them would 
be neutralized by touching them by the hand. 

Replace the jar in its coating, and put the inner coat- 
ing in place. Discharge the jar by making contact be- 
tween the coatings, and it will be seen that there is nearly 
as much electricity in the jar now as if the coatings had 
not been disturbed. 

It is evident then that when the inner and outer coat- 
ings were removed that the electricity in the jar was not 
disturbed, for the outer charge remained bound after 
the inner coating was removed. 

We explain this by saying that the charges are really 



46 ELEMENTARY ELECTRICITY 

on the inner and outer surfaces of the glass and that the 
tin foil is only a means of getting the charge to the glass. 
When we took the jar apart the slight shocks were due 
to leaks, but the main charges stayed on the glass. 

Question 45. Could the glass be discharged while out 
.,>f its coatings? 

Answer. Yes, by connecting inside and outside by a 
J#road strip of tin foil. Doing this as if you were trying 
to wipe off the charge. 

Holding the glass under a tap and running water over 
it will instantly discharge it. 

Question 46. Will you get a spark by this discharge ? 

Answer. It depends. If you make a broad contact at 
fn it you will not, because the charge flows over such a 
lavge surface. If you get a spark it is a feeble one for 
the glass being an insulator it cannot rush to the point of 
discharge quickly enough to make a heavy spark. 

Question 47. Do the foil coatings enable a spark to be 
obtained ? 

Answer. Yes, without them the discharge is a fast 
leak; with them the charge can rush through the con- 
ducting coatings to the discharging tongs and the dis- 
charge is a quick snappy one. 

Question 48. Why will a leyden jar give a second' 
spark a few seconds after being discharged? 

Answer. We believe that the charges penetrate the 
glass to a certain extent and the first spark discharges all 
the charge on the surface of the glass, but it is over be- 
fore the charge which soaked into the glass has time to 
escape. 

The second time the jar is discharged we allow this 
residual charge, which has come back to the surface, a 



STATIC ELECTRICITY 47 

chance to escape. This second spark is very much weaker 
than the first one. 

Question 49. What is meant by saying that the dis- 
charge of a leyden jar is oscillatory ? 

Answer. In the first place the spark or discharge is 
not instantaneous although very quick. This can be 
shown by flashing a spark at some gunpowder, it will not 
explode, but insert a piece of wet string about a foot long 
in the wire leading up to the spark gap and the spark 
will be forced to travel slower. It will then in its slower 
movement heat up the gunpowder and ignite it. 

But this is not all. The spark as we call it, whether it 
be a fast or slow one, is not a single spark, but a series 
of sparks. 

There are always a great number of them jumping 
in alternate directions, each weaker than the last until 
they are too feeble to jump across the gap. The electric- 
ity which forms the spark surges or oscillates between the 
sparking points. 

The sudden emptying of a barrel of water in a tank 
will cause the water in tank to surge back and forth, in 
waves, each successive one being smaller, until the whole 
mass settles down and becomes quiet. 

The action of a pendulum set in motion and gradually 
coming to rest illustrates the action also, provided you 
imagine the pendulum moving at a furious speed. 



LESSON 4. 
Condensers. 

Question 1. Why is a leyden jar often referred to as 
a condenser ? 

Answer. Because that is the general name for ap- 
paratus designed to collect charges of electricity. 

Question 2. What is a condenser ? 

Answer. A condenser consists of two metal plates 
separated by an insulator. 

Question 3. What is the dielectric of a condenser? 

Answer. It is the special name given to the insulator 
of a condenser. 

Question 4. Explain the action of a condenser. 

Answer. If a pane of glass be set on edge and a 
piece of tin foil pasted on one side it will have a certain 
capacity for electricity, but if another piece is pasted on 
the other side and charged with the opposite kind of 
electricity then the first piece will have a greater capac- 
ity. It is as if this arrangement could condense elec- 
tricity, that is get more in the same space. 

Question 5. What is meant by capacity? 

Answer. A piece of tin foil could have its charge in- 
creased until the leakage equalled the amount put on, then 
we might say that it was full, and that the amount on it 
was its capacity. Since this amount would vary with the 
dampness of the air, the temperature of the tin foil, and 
the rate at which you added electricity, a more definite 
way has been adopted to define Capacity, 

48 



CONDENSERS 4§ 

A certain pressure is called a volt. We apply electric- 
ity to the condenser increasing the pressure until it be- 
comes one volt, then we say the condenser is full enough 
and the charge then in it is said to be the capacity of the 
condenser. 

Question 6. On what does the capacity of a condenser 
depend ? 

Answer. On three things :- — 

t. The area of the metal part. 

2. The thinness of the dielectric. 

3. The kind of dielectric. 

Question 7. Does not the kind of metal used or its 
thickness affect the capacity? 

Answer. No, only the area. The greater the area the 
greater the capacity. 

Question 8. Why is it that the thinness of the dielec- 
tric affects the capacity? 

Answer. Because the thinner the dielectric the nearer 
the metal portions are to each other and so the electri- 
cal action between the charges is greater. 

Question 9. Why should the material used as a 
dielectric affect the capacity? 

Answer. Exactly why is not known, but it is a fact 
that the electric action takes place through the same 
thickness of different materials with more or less strength 
according to the material. 

Question 10. What is the metal portion of a condenser 
usually made of? 

Answer. It is always made of tin foil, for it is thin 
and hence light in weight for large areas. 

Question II. What is used generally as a dielectric? 

Answer. For condensers used in. commercial work, 
paper soaked in paraffin is used while for standard con- 



50 ELEMENTARY ELECTRICITY 

densers used in laboratories to test others, mica split into 
thin sheets is used. 

Question 12. What are the advantages of the paper 
condensers, as they are called? 

Answer. They are cheap to make, light in weight 
and good enough for many purposes. 

Question 13. What are the objections to paper con- 
densers ? 





Fig. 12. Paper Condenser. Fig. 13. Mica Condenser. 

Answer. They cannot hold a charge as long as a mica 
condenser, as the insulation is not so good. They leak 
considerably, that is the charge leaks from foil to foil, 
thus discharging the condenser. If rapidly charged and 
discharged the paraffin is heated and may soften or even 
melt. 

Question 14. What are the advantages of the mica 
condensers? 

Answer. They hold their charge without leaking for 
a long time, rapid charge and discharge does not heat 
them much, and even so mica is not affected by tempera- 
tures at which paraffin would liquefy. Their capacity 
is great, so a mica condenser is smaller than a paper 
one of the same capacity. 

Question 15. What are the objections to mica con- 
densers ? 



CONDENSERS 51 

Answer. They are expensive, and while small in size 
are very heavy. 

Question 16. How would the capacities of three simi- 
lar condensers of mica, paraffined paper, and glass com- 
pare? 

Answer. Selecting a condenser with air for a dielec- 
tric as a standard because it is the poorest condenser of 
all, the mica is the best being six to eight times as good 
as the air condenser. The glass one would be three 
times as good as the air one. The paraffined paper 
dielectric makes the poorest condenser being only twice 
as good as air. 

It must be remembered that some mica, such as used 
in oil and gas stoves is worthless as a dielectric, as it 
has fine lines of metal running through it, and hence is a 
conductor. 

Some grades of glass are also almost useless for a con- 
denser. 

Question 17. What is the standard capacity? 

Answer. The scientists' standard was a metal sphere 
of 1 cm * radius perfectly isolated and insulated. 

This capacity is so absurdly large that all electricians 
used as a standard a unit which is 1-900000 of the other. 

This unit is called a microfarad (abbreviated m. f.) and 
is now used by scientists and electricians. 

Question 18. How big a condenser is a microfarad? 

Answer. A mica condenser containing 3600 square 
inches of tin foil. 

Question 19. How is a paper condenser made? 



* The centimeter (abbreviated cm) is the 1/100 part of the 
French standard length called the meter. A meter is approxi- 
mately 39.3 inches, so a centimeter is about 0.4 inches and 
there are roughly 2 x /z cm. to one inch. 



52 ELEMENTARY ELECTRICITY* 

Answer. The finest and thinnest linen paper is ex- 
amined to be sure that it is free from small holes, the 
tiniest hole causes a sheet to be rejected. 

They are then dried and warmed, dipped in a bath of 
melted paraffin, from which all water has been extracted, 
and allowed to drain and cool. 

A pile is made of alternate sheets of tin foil and papen 
The papers are placed with all their edges even, but each 
alternate foil projects on the same side of the pile. The 
first, third, fifth foils project to the right and all the even 
numbered foils project to the left. 

Each set are all connected together, and the whole mass 
clamped tightly and put in a case for protection. 

Binding posts are connected to each set of foils. 

Question 20. What name is given to the quality of 
a dielectric? 

Answer. Dielectric capacity or specific inductive 
capacity is the name used. 

Question 21. What is the standard of dielectric ca- 
pacity ? 

Answer. Perfectly dry air at normal pressure and 
temperature of o° Centigrade* is said to have a dielec- 
tric capacity of 1. 

Question 22. How is the dielectric capacity of ma- 
terials measured? 

Answer. By comparing them with air. Air has the 
least value as a dielectric, so other materials are said to 



* The Fahrenheit thermometer is the standard throughout the 
business and social life of the United States and Great Britain. 

The temperature of freezing water is called 32 degrees and 
that of boiling water 212 degrees. The range between these 
temperatures is divided into 180 equal parts, numbered con- 
secutively from 33 to 211, and as far below 32 and above 212 



CONDENSERS 53 

as is needed these equal divisions are carried. If carried below 
o, we read the temperatures as minus I, minus 2, or I below 
zero, 2 below zero; and we write them — I, — 2. 

In scientific work all over the world, the thermometers are 
marked differently. 

The freezing point of water is marked o and the boiling 
point of water ioo, and the space between into ioo equal parts. 
These divisions are carried down below the zero and above 
the ioo mark. 

This thermometer was introduced by a man named Celcius, 
but is named Centigrade because it has ioo steps between 
freezing and boiling points. (Centum is ioo in Latin and 
gradus means step.) 

Since ioo° Cent. = 212° Fah. 
and o° Cent. = 32 Fah. 
then ioo° Cent. = 180 Fah. 
asid 1 Cent, degree = 1.8 Fah. degree. 
To change Centigrade readings to Fahrenheit: 

Multiply by 1.8 and add 32. 
Ex. — A room whose temperature is 21 ° C. is 6g°.2 Fah. 

21 X 1.8 = 37-2 + 32 = 69.2. 
To change from Fahrenheit tp Centigrade: 

Subtract 32. Multiply by 5 and divide by 9. 
A room which is 70 Fah. is also 21 ° (approx.) Cent. 
70 — 32 = 38 

38X5 = 190 ioo-H9 = 2i°.i Cent. 

The mark ° means degree and the abbreviation C. or Fah. 
after it tells the kind of degree. 

When a decimal fraction is written it is always thus: 7°.2, 
so that should the decimal point be forgotten or not written 
clearly it can not be mistaken for 72 . 

Also when a fraction of a degree is written it is thus: o°.5, 
so that we may know that it is five-tenths of a degree and not 
5 degrees even if decimal point is not there. 

The is placed there to show that it really is five-tenths and 
that some figure has not been forgotten. 

It might be that 2°. 5 was meant and °.s written by an omis- 
sion. By writing o°.5 the writer shows he has not forgotten a 
figure. 



54 ELEMENTARY ELECTRICITY 

have a dielectric capacity of 2 or 3 as the case may be, if 
they are twice or three times as good as air, when used 
in a condenser. 

Question 23. Has capacity any effect on commercial 
work ? 

Answer. Yes a great deal. 

Question 24. Mention some effects. 

Answer. In sending alternating currents through a 
long line the presence of capacity may help to neutral- 
ize some of the bad effects of coils of wire in the circuit.* 

Also in telephone lines the presence of much capacity 
is exceedingly bad, making the transmission of the voice 
difficult and producing a tinny tone. 

For this reason paper insulation is used in telephone 
cables, rather than rubber. The latter has a much higher 
dielectric capacity, making the cable a better condenser 
and a worse telephone line. 

Question 25. Are there any other effects? 

Answer. Yes. The capacity of long telegraph lines 
makes signalling very slow ; for the line has to be filled 
up each time before the signal will be transmitted, and 
it has to empty itself before the next signal can begin. 

Long submarine cables are especially troubled this 
way. The wire inside and the water outside are the two 
conductors and the gutta perchaf insulation is the 
dielectric. This makes a condenser. 



*A circuit is the path of the current from the battery or 
generator out and back again to the starting point. By line 
we often mean the same thing. 

f Gutta percha is a gum something like rubber but a better 
insulator, less dielectric capacity, less likely to deteriorate 
with age, and can stand more moisture. It is injured by light, 
and should never be used in a light, hot, dry place. It is 



CONDENSERS 



55 




Fig. 14. Two Condensers wired so as to be in Parallel. 



One of the Atlantic cables has a capacity of about 
iooo m. f. (microfarads). This makes the signalling 
very slow and limits the amount of business that can be 
transacted per hour. . 



easily softened by placing in hot water. It is lighter than 
rubber and a piece of it will usually float. 
It is used chieflv as an insulator in ocean cables. 



56 ELEMENTARY ELECTRICITY 

The operators can send faster than the cable can take ; 
that is if the operators went as fast as they could, the 
signals would jumble up and be unintelligible at receiving 
end. 

Question 26. How are condensers used in commercial 
work ? 

Answer. In ocean cable telegraphy a condenser is 
often put in at each end, which by absorbing the electric- 
ity on the opening of the circuit and giving it out on the 
closing of the circuit, help the battery to fill the cable on 
the close of the telegraph key, and help to stop the elec- 
trical flow when key is opened. 




Fig. 15. Diagram of Two Condensers connected in Parallel. The 

black lines represent the sheets of tin foil. 



In ordinary telegraphy when sending two or four mes- 
sages, over the same wire at the same time, an artificial 
line must be formed which is electrically exactly like the 
real line. Coils of wire give the resistance of the actual 
line, and condensers its capacity. 

Question 27. How can two condensers be used as one 
large condenser? 

Answer. As in Fig. 14, connect a wire from one bind- 
ing post of the first condenser to either binding post of 
the other condenser. Connect the other two posts by a 
wire. Connect the wires of the circuit, one to each of 
the wires joining the condensers. 



CONDENSERS 



57 



A convenient way of attaching the wires of the circuit 
is to loosen up both binding posts on the one condenser ; 
slip the circuit wires under and tighten up again. 



To Circuit 




To Circuit 



Fig. 16. Two Condensers in Series. 

Question 28. What is the actual capacity of two con- 
densers connected in parallel ? 

Answer. The capacity of this arrangement is the 
sum of the two capacities. 



< 

A- 



-*— Mr-H 



K, 



K, 



Pig. 17. Diagram of Two Condensers in Series. K, is supposed to 

have a greater capacity than K 2 . The dotted lines 

show where to connect voltmeters V l & V, 

to read pressure of condensers. 



Question 29. Suppose you connect the circuit (or 
line) through the two condensers? 

Answer. If as shown in Fig. 16 you bring a line wire 
to one binding post of the first condenser, and the other 



58 ELEMENTARY ELECTRICITY 

line wire to a post of the other condenser, and join the 
other two posts with a wire you will have them in series. 

Question 30. What is the capacity of two condensers 
in series? 

Answer. The actual capacity of the two in series will be 
ihc product of the two divided by their sum. 

Calling Q and C 2 the capacities of the condensers, the 

C XCo 

joint or combined capacity will be expressed as * 2 

CtTVJ 

Ex. : — A 2 m f and a 3 m f condenser are connected in 
series. What is capacity of combination? 

d+C* 2+3 5 1 - mf - 
You will notice that the capacity of the two wired up 
in this way is less than the capacity of either. 

Question 31. Suppose you connect three condensers or 
even more in parallel. What is their joint capacity? 
Answer. The sum of their separate capacities. 
Question 32. Suppose you connect three or more con- 
densers in series, or as it is sometimes called in cascade. 
What is their joint capacity? 

Anszcer. Divide each separate capacity into 1 add the 
answers and divide this into 1. Result is joint capacity. 

Suppose three condensers have capacities of ^, 3 and 
5mf. 

% into 1 goes 2 equals fl 
3 into 1 goes J equals -rs 
5 into 1 goes j- equals ^ 
Sum U 
-ff into 1 goes H times. 
-g4 or 0.4 (approx) m f. 
Question 33. Describe an experiment showing how a 
condenser works, 



CONDENSERS 



59 



Answer. Suppose as in Fig. 18 we have two metal 
disks A and B insulated by glass supports, with a sheet 
of glass or mica between them. 

Let B be connected by a wire to the knob of an electric 
machine and let A be joined to a gas or water pipe by a 
wire ; thus connecting it to ground or earth. 

The -f- charge from machine will act by induction 
across the dielectric C, on A and repel + to earth, leav- 
ing the disk A — ly charged. 

This — charge will react on B and draw more + from 
the machine. 



Fig. 18. A Condenser arranged so as to use different dielectrics at 
C, and with plates A & B movable. 



The nearer A and B are together the better the in- 
duction acts and the more electricity will be condensed. 

If the wires be removed from A and B and the disks 
drawn apart, the pith balls will fly out showing that there 
is more electricity "free" to act than before. 

We ought not to say more electricity is present, it is 
simply more "free" ; for the two charges will not hold 
each other so "bound" at the greater distance. This 
freed electricity spreads over the plate and balls. When 
the disks approach each other again this free electricity is 
drawn back to the plates and held bound. Hence the 
pith balls become discharged or nearly so and fall. 



LESSON 5. 
Electrical Machines. 

Question I. What is an electrical machine? 

Answer. Used in this sense the words mean any of 
the machines capable of producing static electricity. 

Question 2. Describe the simplest machine. 

Answer. The simplest machine is a friction machine. 
A circular glass plate is mounted on an axle and arranged 
so as to be turned rapidly by belt and pulley. See Fig. 19. 

At the top and bottom of plate a cushion of curled hair 
covered with leather is bent around so as to squeeze the 
plate. Light springs keep these cushions in firm contact. 

At both sides of the plate is a set of spikes nearly 
touching the plate both on the back and the front. A con- 
ductor connects the two sets of spikes. A wire from this 
conductor leads to a metal knob or club which is called 
the prime conductor. 

The two cushions are connected by a wire, and this in 
turn to the ground. 

A silk bag or flap runs from the cushion or rubber to 
the spikes or comb. Both the rubber and the comb are 
insulated by glass supports. 

The rubber has a thin layer of tallow spread on it and 
some powdered electrical amalgam sprinkled on. They 
are- then pressed against the glass and the springs ad- 
justed to keep them there. 

When the plate is rapidly '/evolved the friction between 
the glass and the amalgam coated surface of the rubber 

60 



ELECTRICAL MACHINES 61 

produces electrification; a + charge on the glass and a 
— charge on the rubber. 

Positive electricity flows from earth to the rubber and 
neutralizes its charge. In fact the ground wire keeps the 
rubber continually neutral. 




Fig. 19. Simple Electrical Machine. 

The + charge is carried around on the glass in -front 
of the comb which is connected with the prime conductor 
repelling a + charge to the knob of the conductor and 
attracting the — into the comb. The effect of the spikes 
is to emit a — ly charged electrical wind which neutral- 
izes the glass plate and prepares it for the action of the 
next rubber and at the same time leaves the prime con- 
ductor +ly charged. 

Question 3. What is electrical amalgam ? 

Answer. One ounce of tin and one ounce of zinc are 



62 



ELEMENTARY ELECTRICITY 



melted together and while melted four ounces of mercury 
are stirred in. When cool the mass is powdered and 
sifted. It may be sprinkled on the rubber from a salt 
shaker. 

Question 4. Why is this amalgam used? 

Answer. It produces a better charge than any other 
substance and moreover, by being a conductor helps the 
prompt neutralization of the rubber, which also tends to 
make the charge on the glass plate larger. 




F g. 20. Electrical Winds. 

Question 5. Is the use of this amalgam necessary? 

Answer. No. Powdered graphite will work very well. 
Simply rub it into the leather of the cushion. Of course 
omit the tallowing. 

Question 6. What is an electrical wind? 

Answer. It has been found that electricity leaks from 
sharp cornered bodies like a cube faster than from 
rounded ones like a ball. From sharp pointed bodies it 
leaks so fast as to actually produce a brisk air current. 

If a needle be fastened to the prime conductor and a 
lighted candle held near the needle the wind rushing off 
the needle will blow the candle flame aside. See Fig. 20. 

The wind can also be plainly felt by the hand, 



ELECTRICAL MACHINES 63 

Question 7. Why does the candle on the knob get 
blown in the opposite direction? 

Answer. Because now the wind is caused by the op- 
posite kind of electricity flowing off the needle to the 
knob. 

The wind is always blowing off the point. 

Question 8. What are the silk bags for? 

Answer, The silk being an insulator prevents the + 
charge on the glass from leaking off into the air before 
it arrives at the comb. 

It is believed by many that the air currents produced 
by the swiftly moving plate, electrifying the silk nega- 
tively and being a non-conductor, it is imperfectly neu- 
tralized by any ground connection that may happen to 
exist, so there is always a — charge on the silk to "bind" 
the + charge on the glass. 

This action is not strong enough to interfere with the 
effect of the negative wind at the comb. 

Question 9. Are not frictional machines generally un- 
reliable ? 

Answer. Yes. Dampness and dust may prevent them 
from working. Glass attracts moisture so that the ma- 
chines always have to be heated to dry them before use. 

The amalgam will need renewing before use if the 
machine has been standing idle for a couple of months. 

Question 10. Is there another type of electrical ma- 
chine more reliable? 

Answer. Yes, the influence machine. These are very 
reliable. 

Question II. What principle do they involve? 

Answer, The principle of charging induction or in- 
fluence, and of doubling up charges. 



64 ELEMENTARY ELECTRICITY 

Question 12. Explain what is meant by charging by 
influence ? 

Answer. A body touched while under the influence of 
a charge acquires a charge of the opposite kind. 

Question 13. What is meant by doubling up charge? 

Answer. Suppose one (A) of two insulated conduc- 
tors (A and B) is charged ever so little with say + elec- 
tricity. Let a third insulated conductor, which we will 
call a carrier be arranged so as to move back and forth 
between A and B. 

Let C be touched with finger while near A. 

It will acquire a small — charge. Move it over and 
make contact with B which will receive some — elec- 
tricity. Move C a short distance from B and touch it. 
C will acquire a + charge by influence. 

Move C over to A and let them make contact which 
will give some more + electricity to A. Move C away a 
short distance and touch it. This charges C with — elec- 
tricity. Move C over to B and make contact which in- 
creases the — charge on B. 

Keep this up and the charges on A and B keep increas- 
ing and by acting more strongly on C they make the in- 
crease a rapid one. 

Question 14. What machines work on these princi- 
ples? 

Answer. The Toepler machine which has been per- 
fected by Holtz and Voss, and the Wimshurst machine. 

Question 15. Describe the Toepler machine. 

Answer. The principle of the machine is described by 
Silvanus Thompson. 

Before describing some special forms we will deal with 
a generalized type of machine having two fixed field- 



ELECTRICAL MACHINES 



65 



plates, A and B, which are to become respectively + and 
— , and a set of carriers, attached to a rotating disk or 
armature. Figure 21 gives in a diagrammatic way a view 
of the essential parts. For convenience of drawing it is 
shown as if the metal field-plates A and B were affixed 
to the outside of an outer stationary cylinder of glass; 




Pig. 21. Diagram to show the principles upon which the Toepler 
Machine operates. 



the six carriers p, q, r, s, t, and u being attached to the 
inside of an inner rotating cylinder. The essential parts 
then are as follows : 

(I) A pair of field-plates A and B. 
(II) A set of rotating carriers p, q, r, s, t, and u. 
(Ill) A pair of neutralizing brushes n lf n 2 made 
of flexible metal wires, the function of 
which is to touch the carriers while they 
are under the influence of the field-plates. 
They are connected together by a diagonal 
conductor, which need not be insulated. 



66 ELEMENTARY ELECTRICITY 

(IV) A pair of appropriating brushes a r a 2 , which 
reach over from the field-plates to appro- 
priate the charges that are conveyed 
around by the carriers, and impart them to 
the field-plates. 

(V) In addition to the above, which are sufficient 
to constitute a complete self-exciting ma- 
chine, it is usual to add a discharging ap- 
paratus, consisting of two combs c 1 , c 2 , to 
collect any unappropriated charges from 
the carriers after they have passed the ap- 
propriating brushes; these combs being 
connected to the adjustable discharging 
balls at D. 

The operation of the machine is as follows. The neu- 
tralizing brushes are set so as to touch the moving car- 
riers just before they pass out of the influence of the field- 
plates. Suppose the field-plate A to be charged ever so 
little positively, then the carrier p, touched by n, just as it 
passes, will acquire a slight negative charge, which it will 
convey forward to the appropriating brush a. v and will 
thus make B slightly negative. Each of the carriers as it 
passes to the right over the top will do the same thing. 
Similarly each of the carriers as it passes from right to 
left at the lower side will be touched by n 2 while under 
the influence of the — charge on B, and will convey a 
small -f- charge to A through the appropriating brush a 2 . 
In this way A will rapidly become more and more +, 
and B more and more — ; and the more highly charged 
they become, the more do the collecting combs c 1 and c 2 
receive of unappropriated charges. Sparks will snap 
across between the discharging knobs at D. 



ELECTRICAL MACHINES 



CI 



The machine will not be self-exciting unless there is a 
good metallic contact made by the neutralizing brushes 
and by the appropriating brushes. If the discharging 
apparatus were fitted at c v c 2 with contact brushes in- 
stead of spiked combs, the machine would be liable to lose 
the charge of the field-plates, or even to have cht ir 
charges reversed in sign whenever a large spark was 
taken from the knobs. 

It will be noticed that there are two thicknesses of glass 
between the fixed field-plates and the rotating carriers. 
The glass serves not only to hold the metal parts, but 
prevents the possibility of back-discharges (by sparks or 
winds) from the carriers to the field-plates as they pass. 

Toepler's Influence Machine. — In this machine, as con- 
structed by Voss, are embodied various points due to 
Holtz and others. Its construction follows almost literally 
the diagram already explained, but instead of having two 
cylinders, one inside the other, it has two flat disks of 
varnished glass, one fixed, the other slightly smaller ro- 
tating in front of it (Fig. 22). The field-plates A and B 
consist of pieces of tinfoil, cemented on the back of the 
back disk, each protected by a coating of varnished paper. 
The carriers are small disks or sectors of tinfoil, to the 
number of six or eight, cemented to the front of the front 
disk. To prevent them from being worn away by rubbing 
against the brushes a small metallic button is attached tc 
the middle of each. The neutralizing brushes n 2 , n 2 are 
small whisps of fine springy brass wire, and are mounted 
on the ends of a diagonal conductor Z. The appropriat- 
ing brushes a lf a 2 are also of thin brass wire, and are 
fastened to clamps projecting from the edge of the fixed 
disk, so that they communicate metallically with the two 



68 



ELEMENTARY ELECTRICITY 



field-plates. The collecting combs, which have brass spikes 
so short as not to touch the carriers, are mounted on in- 
sulating pillars and are connected to the adjustable dis- 
charging knobs D 1? D 2 . These also communicate with 




FRONT ROTATING DISK 
WITH CARRIERS ON FRONT. 



Fig. 22. A Toepler Electrical Machine. 



the two small Leyden jars J r J 2 , the function of which 
is to accumulate the charges before any discharge takes 
place. These jars are separately depicted in Fig. 22. 
Without them, the discharges between the knobs take 



ELECTRICAL MACHINES 6iJ 

£>lace in frequent thin blue sparks. With them the sparks 
are less numerous, but more brilliant and noisy. 

To use the Toepler (Voss) machine first see that all the 
four brushes are so set as to make good metallic contact 
with the carriers as they move J>ast, and that the neutral- 
izing brushes are set so as to touch the carriers while 
under influence. Then see that the discharging knobs 
are drawn widely apart. If it is clean it should excite 
itself after a couple of turns, and will emit a gentle hiss- 
ing sound, due to internal discharges (visible as blue 
glimmers in the dark), and will offer more resistance to 
turning. If then the knobs are pushed nearer together 
sparks will pass across between them. The jars (the ad- 
dition of which we owe to Holtz) should be kept free 
from dust. Sometimes a pair of terminal screws are 
added at S 1? S 2 (Fig. 22) connected respectively with the 
outer coatings of the jars. These are convenient for at- 
taching wires to lead away discharges for experiments at 
a distance. If not so used they should be joined together 
by a short wire, as the two jars will not work properly 
unless their outer coatings are connected. 

Question 16. Describe the Wimshurst machine. 
Answer. Silvanus Thompson describes it as follows : 
In this, the most widely used of influence machines, 
there are no fixed field-plates. In its simplest form it con- 
sists of (Fig. 23) two circular plates of varnished glass, 
which are geared to rotate in opposite directions. A num- 
ber of sectors of metal foil are cemented to the front of 
the front plate and to the back of the back plate ; these 
sectors serve both as carriers and as inductors. Across 
the front is fixed an uninsulated diagonal conductor, 
carrying at its ends neutralizing brushes, which 



70 



ELEMENTARY ELECTRICITY 



touch the front sectors as they pass. Across the 
back, but sloping the other way, is a second diag- 
onal conductor, with brushes that touch the sec- 




Fig, 



Wimshurst Electrical Machine. 



tors on the hinder plate. Nothing more than this is 
needed for the machine to excite itself when set in rota- 
tion ; but for convenience there is added a collecting and 
discharging apparatus. This consists of two pairs of in- 
sulated combs, each pair having its spikes turned inwards 



ELECTRICAL MACHINES 



W 



toward the revolving disks, but not touching them; one 
pair being on the right, the other on the left, mounted 
each on an insulating pillar of ebonite. These collectors 
are furnished with a pair of adjustable discharging knobs 
overhead ; and sometimes a pair of Leyden jars is added, 
to prevent the sparks from passing until considerable 
quantities of charge have been collected. 




Fig. 24. The Wimshurst Machine laid out in diagrammatic way, to 
show principle of its operation. 



The processes that occur in this machine are best ex- 
plained by aid of a diagram (Fig. 24), in which, for 
greater clearness, the two rotating plates are represented 
as though they were two cylinders of glass, rotating op- 
posite ways, one inside the other. The inner cylinder will 



*72 ELEMENTARY ELECTRICITY 

represent the front plate, the outer the back plate. In 
Figs. 23 and 24 the front plate rotates right-handedly, the 
back plate left-handedly. The neutralizing brushes n^ 
n 2 touch the front sectors, while n 3 , n^, touch against the" 
back sectors. 

Now suppose any One of the back sectors represented 
near the top of the diagram to receive a slight positive 
charge* As it is' moved onward toward the left it will 
come opposite the place where one of the front sectors 
is moving past the brush n x . The result will be that the 
sector so touched while under influence by n 1 will acquire 
a slight negative charge, which it will carry onward 
toward the right. When this negatively-charged front 
sector arrives at a point opposite n 3 it acts inductively on 
the back sector which is being touched by n 3 ; hence this 
back sector will in turn acquire a positive charge, which 
it will carry over to the left. In this way all the sectors 
will become more and more highly charged, the front 
sectors carrying over negative charges from left to right, 
and the back sectors carrying over positive charges from 
right to left. At the lower half of the diagram a similar 
but inverse set of operations will be taking place. For 
when n x touches a front sector under the influence of a 
positive back sector, a repelled charge will travel along 
the diagonal conductor to n 2 , helping to charge positively 
the sector which it touches. The front sectors, as they 
pass from right to left (in the lower half), will carry 
positive charges, while the back sectors, after touching n 4 , 
will carry negative charges from left to right. The 
metal sectors then act both as carriers and as inductors. 
It is clear that there will be a continual carrying of posi- 
tive charges toward the right, and of negative charges to 
the left. At these points^ toward which the opposite kind 



ELECTRICAL MACHINES 73 

of charges travel, are placed the collecting combs com- 
municating with the discharging knobs. The latter ought 
to be opened wide apart when starting the machine, and 
moved together after it has excited itself. 




Fig. 25. Electrical Wheel. 

In larger Wimshurst influence machines two, three, or 
more pairs of oppositely-rotating plates are mounted 
within a glass case to keep off the dust. If the neutraliz- 
ing brushes make good metallic contact these machines 
are all self-exciting in all weathers. Machines with only 
six or eight sectors on each plate give longer sparks, but 
less frequently than those that have a greater number. 
Mr. Wimshurst has designed many influence machines, 
from small ones with disks 2 inches across up to that at 
South Kensington which has plates 7 feet in diameter. 



74 



ELEMENTARY ELECTRICITY 



Prior to Wimshurst's machine Holtz had constructed 
one with two oppositely-rotating glass disks ; but they had 
no metal carriers upon them. It was not self-exciting. 

Question 17. Give some experiments showing the ac- 
tion of electricity. 

Answer. Example 1. If a pivot be erected on the 
knob of an electric machine and a small wheel with wire 
spokes bent as shown in Fig. 25 is balanced on the pivot, 
the electrical winds coming from the pointed ends will 
drive the wheel around. 




Fig. 26. Puncturing a Card with Spark from a Leyden Jar. 



Example 2. A card may be punctured as shown in 
Fig. 26. There will be a burr on both sides of the hole in 
the card as if the material were pulled out from the card 
on both sides at the same time. 



ELECTRICAL MACHINES 



75 



Example 3. A fine wire melted as shown in Fig. 27. 

Example 4. When an electrical machine is actuated in 
the dark, accompanying the slight crackling which indi- 
cates leaking, at several points on the frame may be seen 
luminous appearances, called brushes ; and if a conductor, 
a wire, or the hand, be presented toward the terminal of 




Fig. 27. Melting a Wire with a Battery of Leyden Jars. 



the machine, just beyond the striking distance of a spark, 
one of these brushes will reach for the object so pre- 
sented. The brush discharge consists of a short stalk, 
from which spreads, a shape not unlike a palm leaf fan, 
consisting of rays which become thinner and lighter 
towards their outer extremity. 



76 ELEMENTARY ELECTRICITY 

Example 5. If a doll's head having hair, be placed on 
the terminal of the machine, and the machine actuated, 
the hair will tend to straighten out in all directions, and 
will reach for the hand or other conductor presented. 
Discharging the machine by placing its terminals in con- 
tact, will restore the hair to its normal condition. 

Example 6. A human leyden jar may be made by a 
person occupying a stool or chair, the legs of which are 
standing in dry India rubber overshoes, in tumblers, or in 
telegraph insulators. In this position the human leyden 
jar is capable of being charged, and of giving shocks to 
parties standing on the floor or ground. The hair of the 
human jar will stand on end if the charge is considerable, 
and be attracted by the approach of any conductor. The 
charge may be silently discharged through a fork or 
needle held in the hand. 

Example 7. Attach a rod or heavy wire to the terminal 
of the machine, having the curved shape of a shower bath 
standard, and terminating in a metal band, the lower edge 
of which is fitted with points like an inverted crown. One 
sitting or standing beneath such an attachment will feel a 
very perceptible breeze. 

Example 8. Approach the knob of a machine with a 
sharp needle held in the hand, and the discharge will be 
noiseless and not unpleasant. If in a darkened room, the 
discharge will be seen to resemble a blue flame. 

Question 18. It is said that static electricity is only 
on the surface of charged bodies. Is this true ? 

Answer. Yes, as is shown by this experiment. 

On the top of a rod of glass which is fastened to a 
sufficiently heavy base, a brass ring is fixed in a vertical 
position. To this ring, much like a minnow or landing or 



ELECTRICAL MACHINES 



77 



butterfly net frame, is attached a fine linen bag, which 
runs down to a point — like an elongated cone. A silk 
thread extends from the apex or point of the cone, in 
each direction, so that the bag may be reversed at will by 
pulling on the one thread and loosing the other. Now, 
when this bag is charged a test shows electricity on the 
outside, and none on the inside of the net, in all cases. 
Reversing the bag reverses the surface electrified, no 
matter how often or how suddenly the change is made. 
See Fig. 28. 




Fig. 28. Static Electricity always stays on Outside of Body. 



Question 19. Is the charge spread evenly over the 
whole surface? 

Answer. Xo. The density of electricity residing on 
the surface of a conductor sufficiently removed from 
bodies affecting it as to be uninfluenced by them, is ma- 
terially dependent as to distribution, on the shape of the 
charged body. For instance, a perfect metallic sphere 



78 



ELEMENTARY ELECTRICITY 



shows the same electrical density over all portions of its 
surface, and while the charge of a metallic disc is hardly 
appreciable on the two surfaces, yet close to the edges 
it increases rapidly to the outer limit of the body. See 
.Fig. 29. 



(C 



3) 







Fig. 29. Distribution of Electrical Charge over the surface of a body, 
showing influence of edges and points. 




*» 



Fig. 30. Charge on a Conductor. Shows Density at Different Points. 



This density increases at all pointed as well as rounded 
extremities. The density is greatest on the most pro- 



ELECTRICAL MACHINES 79 

jecting parts of the surface, or those which have the 
sharpest convexity, while hollows and indentations show 
little or no charge. In consequence of this strain, at a 
sharp projection on a charged conductor, or still more 
markedly, at a point, as in a sharpened wire, the con- 
densation of such an amount of force within such small 
space produces a very rapid escape of electricity from 
such points. For this reason conductors which it is de- 
sired should retain their charge should have no edges or 
points, and must be very smooth. This is why the termi- 
nals of leyden jars and other similar apparatus are in the 
form of knobs and the combs of electrical machines are, 
like lightning rods, pointed, to facilitate silent, rapid 
leaking. 

The density of the charge is also shown by the relative 
repulsion of the pith balls at different points on the sur- 
face, as in Fig. 30. 



LESSON 6. 

Lightning. 

atmospheric electricity. 

The similarity in the effects of lightning and those of 
the electric spark enlisted the minds of the earliest phys- 
ical investigators. Lightning ruptures and disintegrates 
substances opposing its passage, and where these are com- 
bustible, often ignites them. It is capable of producing all 
the effects of heat in melting metals, and volatilizing them, 
and leaves behind it, in many instances, the odor which 
we recognize as that pertaining to ozone. This odor is 
the same that is observed when an electrical machine has 
been working a few minutes. To Franklin is given the 
credit of thoroughly identifying the phenomena of prov- 
ing experimentally with his historic kite, and the aid of 
ley den jars, that, excepting the factors of quantity and 
intensity, the two were one. 

Franklin enumerated the following specific character- 
istics pertaining to, and tending to show that lightning 
and the spark were but different manifestations of static 
electricity : "Giving light ; color of the light ; crooked di- 
rection ; swift motion ; being conducted by metals ; noise 
in exploding; conductivity in 'water and ice; rending im- 
perfect conductors ; destroying animals ; melting metals ; 
firing inflammable substances ; sulphureous smell (ozone) ; 
and similarity of appearance between the brush discharge 
from the tips of masts and spars sometimes seen at sea, 

80 



LIGHTNING 8l 

cailed St. Elmo's fire by the sailors, and the slow escape 
from points on an electrical machine or a leyden jar." 

The cause of electrical charges in the atmosphere is un- 
known, there are half a dozen explanations any one of 
which or all may be correct. 




A Brush Discharge of Lightning. 

It is generally agreed, however, that the cause of light- 
ning is the condensation of water vapor in clouds. 

Thunder Storms. — One of the most interesting mani- 
festations of statical electricity is the thunder shower 
which is brought about in this way. Although bodies can- 



§2 ELEMENTARY ELECTRICITY 

not be charged throughout their substance, the electricity 
being always on the surface of the body ; yet clouds seem 
to be electrified all the way through. This is because a 
collection of little particles of water, like a rain cloud, can 
have a charge on the surface of each particle. 

As the particles of water fall by gravitation many touch 
and unite, so that the charges of say eight small drops 
are now in a drop weighing eight times as much, but 
which has only half the surface of the eight, hence the 
pressure is four times as large. This occurs in the follow- 
ing manner : 

Since the surface of a sphere is equal to the product of 
the square of its diameter and the fraction twenty-two 
sevenths and the volume of a sphere is equal to one-s xth 
of the product of twenty-two sevenths and the cube of the 
diameter, we can calculate as follows : 

Eight spherical rain drops each I mil* in diameter have 
a total surface of 25 sq. mils. They have a total volume 



* Note. — A mil is the name given in machine shops and in all 
electrical work to one-thousandth of an inch. 
The mathematical work of the above is here given in full. 
Twenty-two sevenths is a convenient and quite accurate way 
of expressing the number 3.1416. 
Total surface of eight spheres 

8 X 3.1416 X 1 X 1 =: 25 sq. mils. 
Total volume of eight spheres 

8 X 0.166 X 3.1416 X 1 X 1 X 1 = 4-2 cu. mils. 
Volume of the large sphere = 4.2 cu. mils. 
Diameter of large sphere is the cube root of 

4.2X6-^3.1416 = 8 mils. 
Cube root of 8 = 2 mils. 
Surface of large sphere 

3.1416 X 2X2 = 12.5 sq. mils. 

25 -=- 12.5 = 2. So large sphere has only half surface of 
eight small ones. 



LIGHTNING 83 

of 4.2 cu. mils. Now the sphere composed of the eight 
drops has the same volume i. e. 4.2 cu. mils and we can 
find its diameter from the rule : The diameter of a sphere 
is the cube root of the continued product of its volume, 
six, and seven twenty-seconds. Applying this we find the 
diameter to be 2 mils, hence its surface is 12.5 sq. mils. 
That is exactly one-half the surface of the eight separate 
drops. 

Therefore the eight charges having been squeezed into 
the surface where only two were before, the pressure 
must be fou^ times as great. 

By the repeated union of these larger drops, the pres- 
sure becomes very high, and meanwhile the influence of 
the charged cloud is to accumulate a charge of the oppo- 
site name in the earth under the cloud. This in turn in- 
creases the pressure. When finally the pressure gets high 
enough the air is punctured, and the spark jumps between 
earth and cloud. It literally punches a hole in the atmos- 
phere and the inrush of air to fill the hole causes the loud 
sounding thunder. 

There are two kinds of atmospheric electricity different 
enough to need different devices to guard against their 
effects. Some forms of lightning arresters combine both 
devices in the one piece of apparatus. 

Lines are sometimes struck by lightning. This means 
that an accumulation of electricity suddenly makes con- 
nection with the line, discharging through it, its machin- 
ery and instruments. 

A stroke discharges violently and cannot be discharged 
by degrees, for the line is not strained until the lightning 
strikes. 

Lines are often affected by "static," which means that 






84 ELEMENT' ARY ELECTRICITY 

electricity has accumulated on the line until its pressure is 
high enough to do damage when discharging along the 
line through machinery, etc. 

Static changes can accumulate on long open air lines as 
well as on lead sheathed cables. 

The cable insulator makes a dielectric arid the lead 
cover and copper conductor the two plates of a con- 
denser. 

In the open air line the two wires of the circuit form 
the plates and the air between the dielectric. Also the 
two wires together and the earth form two plates with 
air as the dielectirc. 

A transmission line 150 miles long may have a capacity 
of 3 mf, i. e. 1 mf per 50 miles. 

Both these effects, static and strokes are summed up in 
the one word "lightning." 

Static charges may be discharged little by little as they 
accumulate, so that when properly protected a line never 
has a static charge on it great enough to do any damage. 

STATIC EFFECTS ON CIRCUITS. 

On high voltage alternating current lines not only light- 
ning makes trouble but accidental grounds, and switching 
operations some times cause "static effects." 

This use of the word static is hardly a good one as 
these effects are all due to a wave of electricity flowing 
over the circuit. This wave is the flow of an electro- 
static charge from one point of the circuit to another. 

When a disturbance is created at any point of an elec- 
tric circuit as the sudden opening of a field circuit, an arc 
jumping across the lines, the release of a large "bound" 
static charge, or the striking of lightning, etc., a set of 



LIGHTNING S5 

waves of electricity are started just as when a stone is 
thrown into a narrow stream of smooth water. 

Our troubles are caused by "static" electricity, but are 
actually produced by the wave or surge following the dis- 
turbance. 

The damage done by a surge depends on the condition 
of the circuit, whether dead,* live or loaded, the excel- 
lence of its arresters in design and state of repair. 

We will make this distinction between lightning and 
other static troubles. 

When we say lightning we mean an actual stroke and 
its effects at the point of striking. 

When we say surge we mean any or all static electrical 
troubles on the lines at points where the lightning did not 
strike. 

Lightning can do damage by striking and producing 
a surge at the same time. 

A surge is electricity at very high pressure and very 
great frequency ; the normal current on a line is of mod- 
erate pressure and low frequency. 

The normal current is produced by the generators. 

Surges may be produced by 

1. Switching off live lines from a station. 

2. Switching on dead transmission lines, branch l : nes, 
transformers, or underground cables to a station or to a 
live line. 

3. Short circuits which are sudden. 



*A dead line is one not connected to any source of electricity. 

A live line is one connected to a generator in operation or 
another circuit and has pressure on it ready to deliver power. 
When lamps, motors, etc., are connected to a live line it car- 
ries the current to operate them and is called a loaded line. 



86 ELEMENTARY ELECTRICITY 

4. Grounds or partial short circuits which occur sud- 
denly. 

5. Lightning stroke. 

By high pressure we mean any pressure over 50% 
greater than the line voltage. 

Frequency is best explained as follows : 

If the feed wire of a city trolley line be cut and a pres- 
sure indicator inserted the pointer will stand rather steady 
at about 500 volts. This shows a steady current. 

If, however a feed cable from one of the main power 
stations to a sub-station be cut and a pressure indicator 
inserted (called an oscilligraph), the instrument will 
show that the pressure is constantly and very rapidly 
changing from a high value to a low one, then reversing 
and going down to a high negative value and coming 
back to zero. 

The pressure keeps rising and falling and alternating 
positive and negative. 

It will make from 15 to 33 of these complete changes, 
called cycles, every second. The frequency with which 
these changes occur is called the frequency. 

Frequency is then the number of cycles per second. 

Take a transmission line delivering power at the dis- 
tant end where the capacity is about 3 mf. 

While this line is in operation supplying power, the cur- 
rent varies, according to the load. When all the motors 
it supplies are stopped and all the transformers at the 
other end are cut off there is no load but still about 50 
amperes flow into the line. 

This current is charging the line, that is, keeping up 
the voltage of the line ; for the line is a condenser and 
takes current to charge it. 



LIGHTNING 



87 



If a switch is opened when the line is loaded there 
would be an arc -formed at the switch blades on account 
of the large current broken and the discharge of the line 
itself. 




A Lightning Stroke. 



If a switch is opened when the line is simply alive, that 
is, charged but not loaded, there is a slight arc at the 
switch due to the discharge of the line. 

If the generators are stopped and the line is "dead" of 
course there is then no arc at the switch opening. 

The arc formed by the opening of a loaded line allows 



88 ELEMENTARY ELECTRICITY 

the line to discharge itself across the arc, but when the 
switch to a live line is opened quickly the small arc dies 
out, leaving the line lightly charged. The line will now 
discharge itself at the weakest point along its insulation 
unless provided with arresters to discharge it in a harm- 
less manner. 

The surge of the charge may raise the pressure on this 
cut out* line to double the pressure of the generators. A 
22000 volt line may rise to 44000 volts when suddenly 
cut out. 

When a single phase generator (See Lesson 28) is 
grounded at one terminal and a "live" branch line con- 
nected to its other terminal is cut at the switch-board the 
pressure caused by the surge may rise to four times the 
original pressure. So a 40000 volt line might have one of 
its branch lines rise to 120000 volts. 

The two cables from a single phase generator to the 
switch board are called its terminals. 

When a dead line is "cut in"* its capacity must be filled 
up and there is a sudden rush of current into it. This 
produces a surge along the line and when the line is 
short the pressure may rise to double the normal pres- 
sure. When the line is long it hardly ever rises to quite 
double pressure. 

It is interesting to know that the first dead line switched 
on to the generators has the least rise in pressure, and the 
last switched on the greatest. So the line with the weak- 
est insulation or poorest arresters may be switched on 
first. 



*To cut in a line is to connect it to the generator or to the 
main transmission line; to cut out is to disconnect it. A cut 
out line is one which has been disconnected. 



LIGHTNING 89 

When a "dead" transformer is connected to a live line 
there is a surge, and due to the choking effect of the coils 
in the transformer, this surge only penetrates a short dis- 
tance. The turns of wire near the end of a transformer 
are insulated with extra thickness of material to protect 
them, and arresters should be placed near each transform- 
er to rid the line of the surge. 

PROTECTION FROM LIGHTNING. 

Persons. 

Question I. What precautions should people take dur- 
ing lightning discharges? 

Answer. Do not stand in the open doorway of a build- 
ing or under a single tree in a field. Standing under a 
group of trees is not so bad. Do not stand near a wire 
fence. 

Question 2. Won't steel articles attract lightning? 

Answer. No, nothing attracts lightning, it merely goes 
by the shortest path whose resistance is fairly low. 

Question 3. Is not staying in a locomotive dangerous, 
especially electric ones? 

Answer. No, it is the safest place you can be, as the 
metal is all around and acts as a shield, carrying the dis- 
charge safely past the person. 

Question 4. What is to be done to a person struck by 
lightning? 

Answer. Treated like a person who has been suffo 
cated, and artificial breathing begun at once, as follows . 

Howard's method of producing artificial respiration 
has this advantage over other methods in that it can be 
successfully practiced by a single person, instead of two, 
and at the same time is equally efficacious. 



90 ELEMENTARY ELECTRICITY 

"Place the subject on his back, head down and bent 
backward, arms folded under the head (under no condi- 
tions raise the head from the ground or floor). Place a 
hard roll of clothing beneath the body, with the shoulders 
declining slightly over it. Open the mouth, pull the 
tongue forward, and with a cloth wipe out saliva or 
mucus. Thoroughly loosen the clothing from the neck to 
the waist, but do not leave the subject's body exposed, for 
it is essential to keep the body warm ; kneel astride the 
subject's hips, with your hands well opened upon his 
chest, thumbs pointing toward each other and resting en 
the lower end of the breastbone ; little fingers upon the 
margin of the ribs and the other fingers dipping into the 
spaces between the ribs. Place your elbows firmly against 
your hips, and using your knees as a pivot press upward 
and inward toward the heart and lungs, throwing your 
weight slowly forward for two or three seconds, until 
your face almost touches that of your patient, ending with 
a sharp push which helps to jerk back to your first posi- 
tion. At the same time relax the pressure of your hands 
so that the ribs, springing back to their original position, 
will cause the air to rush into the subject's lungs. Pause 
for two or three seconds, and then repeat these motions 
at the rate of about ten a minute, until your patient 
breathes naturally, or until satisfied that life is extinct. 
If there is no response to your efforts persistently and 
tirelessly maintained for a full hour, you may assume that 
life is gone. 

"Hot flannels, water bottles, bricks, and warm clothing 
will aid in recovery. Warmth should be maintained, but 
nothing must prevent persistent effort as above described 
Stimulants in small quantities may be administered after 
swallowing is possible, and sleep must be encouraged, as 



LIGHTNING 91 

one of the best recuperatives. Get a physician as early as 
possible. 

The treatment of persons shocked by electric light or 
power currents is identical with that for lightning stroke. 

Buildings. 

Question 5. Are lightning rods of any use? 

Answer. Yes, if properly installed they offer a great 
protection. 

Question 6. What material is best for lightning rods ? 

Answer. Copper, as its conductivity is high, and so is 
much lighter, smaller and neater in appearance than an 
iron rod. 

Question 7. What should they weigh ? 

Answer. A copper rod six ounces to the lineal foot 
and an iron rod two pounds per lineal foot. 

Question 8. What kind of rod should be used ? 

Answer. We really do not mean a rod, the word being 
used in a general sense. The best form is a tape or flat 
thin bar. 

Question 9. What kind of tips should be used? 

Answer. They should be pointed. 

Question 10. Why is this? 

Answer. The points tend to discharge the electricity 
of the earth to the air and thus relieve the tension in the 
atmosphere. 

Question II. Where should the rods be placed? 

Answer. Tips should be erected on all parts of build- 
ing projecting above the roof, such as cupolas, chimneys, 
gables. 

Question 12. What should be done to cornices, orna- 
mental iron work, etc.? 



92 ELEMENTARY ELECTRICITY 

Answer. They should all be connected by a soldered 
joint to a "rod." 

Question 13. Should the rod be insulated from build- 
ing ? 

Answer. Xo. It is certainly unnecessary from an elec- 
trical point of view and is troublesome and expensive. 

Question 14. Can the "rod" be run inside the building? 

A .::.:-. Never. This would be very dangerous, as 
lightning if it jumped from rod would surely cause great 
damage. 

Question 15. Must the "rods" be run straight? 

Answer. It is better to run from each point on the 
roof as straight to the ground as possible, avoiding all 
sharp bends, as these give the lightning a chance to jump 
off. 

Question 16. Does each point protect a certain area? 

Answer. No. The amount of space protected by a 
point varies. A sudden rush or disruptive lightning dis- 
charge may strike a building very near a point. Hence 
the more points the greater safely-. 

Question 17. How are the lower ends of "rods" con- 
nected to earth? 

Ansu . . By being well soldered to water pipes or to a 
plate of copper about 3x3 ft. buried in moist earth. 

Question 18. If the rods pass near metal work gas 
pipes, etc., what should be done? 

Answer. Connect them to the rod by wires whose 
joints are well soldered. 

Question 19. Why should the joints be soldered? 

Answer. Because the resistance of an old or badly 
made joint will sometimes cause the lightning to jump off 
the rod. Even' joint in the rod and connections should 
ridered, 



LIGHTNING* 93 

Question 20. How should lightning be prevented from 
entering buildings by the line wires ? 

Answer. Use lightning arresters at the point where 
they enter the building. 

Question 21. What is a lightning arrester? 

Answer. It is a device designed to protect electrical 
apparatus from lightning or atmospheric electricity. 

Question 22. Does it stop the lightning? 

Answer. No, the name arresters is misleading. They 
do not stop the discharges, but turn them aside to a con- 
ductor which leads to the ground. 

Question 23. What circuits need protection? 

Answer. Any circuit which has a part running out 
doors, or any circuit connected to one running into the 
open air. 

Question 24. What kinds of circuits need protection 
most? 

Answer. Long lines, lines running over hills or moun- 
tains. 

Question 25. Does the kind of power on the line af- 
fect the liability of lightning discharge ? 

Answer. No, any kind of line from telegraph to power 
transmission is equally liable to be struck. 

Question 26. Do the station men sometimes disconnect 
the dynamos from the line to prevent the line being 
struck ? 

Answer. No, they do it when they mistrust the light- 
ning arresters' ability to protect the dynamos. Cutting 
the dynamos off puts them in safety, but the line is not 
protected. A dead line is just as apt to be struck as a 
live one. 

Question 27. Are some parts of the country troubled 
with lightning more than others ? 



94 ELEMENTARY ELECTRICITY 

Answer. Yes. In the Rocky Mountains lightning dis- 
charges are very numerous and severe. 

Question 28. What trouble may result from lightning 
striking a line ? 

Answer (1) Burning of insulation on wires in instru- 
ments and machines. 

(2) Puncturing insulation of machinery, like 
dynamos or transformers. Either of these destroys the 
insulating value of the material. 

(3) Melting of wires or fusing together met- 
al parts which are in contact. 

(4) Dangerous injuries to persons. 

(5) Fire caused by an arc jumping across in- 
flammable material. 

(6) The insulators are sometimes cracked or 
even splintered. 

(7) Poles are splintered or sometimes shat- 
tered. 

(8) A cable forming a part of the current is 
more likely to have its insulator punctured than any other 
part of the circuit. This is a very troublesome and costly 
thing to repair. 

This applies to underground or underwater cables more 
than to those strung on poles. 

Question 29. Does lightning follow the shortest path 
or the path of least resistance ? 

Answer. Unlike ordinary current electricity, lightning 
usually follows the straight, short path even if of enor- 
mous resistance. 

Question 30. Does a break in the circuit stop light- 
ning? 

Answer. No, it will jump across and go on. 



LIGHTNING . 95 

Question 31. Do coils have any effect on lightning? 

Answer. Yes. Lightning cannot pass readily through 
a coil of wire. It will do so if this is the only path open to 
it, but if there is any other path not containing coils the 
lightning will usually take the path without coils. It 
sometimes jumps from turn to turn of a coil, thus getting 
past without going through. 

Errata : — Illustration on page 102 should read Shunt 
Circuit in lower part instead of Short Circuit. 



LESSON 7. 

Lightning Arresters. 

low voltage. 

Question 1. What is the oldest form of arrester? 
Answer. The saw tooth spark gap of the telegraph of- 
fices. 



r 



LINE 



|VWV| 



± 



"=3F = " GROUND 

Fig. 31. The Saw-tooth Lightning Arrester as used on Telegraph 
Circuits. I and I are the Instruments and A the Arrester. 

Question 2. Describe it. 

Answer. Two brass plates with V-shaped points are 
set close to each other on an insulating base, one plate is 
connected to the line and the other to a ground plate 
buried in the earth. (See Fig. 31.) 

Question 3. How does it operate? 

Answer. The lightning being of an electrostatic na- 

96 



LIGHTNING ARRESTERS LOW VOLTAGE 9? 

ture discharges from points readily, and being of an enor- 
mous pressure is able to jump the air gap between the 
points. The telegraph instruments contain electro mag- 
nets whose coils act as choke coils. 

The lightning has the choice of the path through the in- 
strument coils or across the air gap. It practically always 
takes the air gap and runs to the earth through the 
ground wire. 

Question 4. What objection is there to this type of ar- 
rester when power circuits are to be protected ? 

Answer. The spark caused by the lightning in leaping 
across the air gap forms a conducting path between the 
plates. 

The pressure on the line due to the generators sends 
the current across this path which forms an arc melting 
the edges of the plates. 

This arc grows larger until it conducts enough current 
to "blow"* the fuses in the circuit, which interrupts the 
service. 

The arcing of an arrester is always caused by the cur- 
rent of the line following the sparks due to lightning dis- 
charge. 

Question 5. What is the easiest way of stopping the 
working current from following the lightning discharge? 

Answer. Place small fuses (as in Fig. 32) in the 
ground wires of the arresters before they join to the com- 
mon f ground wire. Then any working current following 
the lightning discharge will blow these fuses instantly, 



♦Melt. 

|A wire acting as a ground wire for several others is called 
a "common" ground wire. 



98 



ELEMENTARY ELECTRICITY 



leaving the main fuses unharmed. This will not intef 

rupt the service. 

Question 6. What is the objection to the arrangement 
Answer. Often the two fuses are blown, the arreste 

is useless and machinery left unprotected, there being n< 

ground connection to conduct the discharge away. 



UNES 




GROUND 



Fig. 32. The Saw-tooth arrester applied to a Dynamo. F, F ar. 
the regular fuses. D, D are the fuses for Arrester Circuit. 



Question J. But the fuses can be replaced ? 

Answer. Perhaps not before the next discharge has 
come. Moreover, a lightning arrester should allow tht 
static charges which accumulate even in clear, dry weath- 
er to escape. These discharges sometimes snap across an 
arrester in the steady stream. 

Question 8. What are some of the better ways of stop- 
ping the passage of the current after the discharge ? 

Answer. There are various ways, some methods put 
out the arc which is conducting the working current, and 



LIGHTNING ARRESTERS — LOW VOLTAGE 99 

some try to prevent an arc or at least make arc very small. 

Question 9. How is the arc put out? 

Answer. By air blast, electromagnetic action, mechan- 
isms for lengthening the gap momentarily as the dis- 
charge passes, also use of non-arcing metals. 

Question 10. How are arcs prevented? 

Answer. Smothering the arc so that it doesn't form 
for lack of air ; insertion of resistance into the discharge 
circuit which weakens the current following discharge so 
that it cannot hold an arc. 

Question 1 1. What should be done if an arrester in a 
station holds its arc ? 

Answer. The arc should be beaten out with a cloth or 
broom, or it should be smothered with sand. 

Dry powder fire extinguishers are very useful for this 
purpose, but water or liquid extinguishers should never 
be used. 

Question 12. Where should arresters be placed? 

Answer. At the point where lines enter or leave any 
building, and at intervals along the line. 

Question 13. Why should they be placed along the 
line ? Will not the protectors at the buildings protect the 
machinery ? 

Answer. It seems to be generally believed now that 
lightning runs along the lines in waves and that at one 
point it may be so weak that it will not jump to groun J 
through a certain arrester but pass on, and the same 
charge a few miles further on, will either be discharge. 1 
through an arrester there or if there is no arrester, d., 
considerable damage. 

Hence all the most exposed places on the line should 
certainly have arresters and a few strung along the line 
will not be wasted. 



100 



ELEMENTARY ELECTRICITY 



Question 14. What is the best arrester? 

Answer. Each kind has its good points, some will not 
work on low pressures, others will not stand the severe 
test of Rocky Mountain use, but are reasonable in price 
and satisfactory in action in the more open and level parts 
of the country. 

Lvxxe 



i\.xres^,ers 



Goc ouxvA Wvc e- 



Y To Gocoutvd, 



Fig. 33. Bank of Lightning Arresters. 



Any arrester is hardly a complete protection unless 
combined with choke coils. (See Lesson 9.) 

Question 15. Into what classes may arresters be di- 
vided as regards to the circuits and apparatus they pro- 
tect? 

Answer. (1) For use when currents are very small 
and voltages moderate as in telephone lines. The instru- 
ments are very delicate and need absolute protection. 

(2) When currents are small and voltage 
moderate as in telegraph and signalling lines. Here the 
apparatus is heavier and less liable to damage. 



LIGHTNING ARRESTERS— LOW VOLTAGE 161 

(3) • Power lines and lightning circuits where 
currents are heavy and voltage moderate, say up to 2500 
volts. 

(4) Power lines, transmission lines where the 
voltage is very high, say from 1 1000 up to 50000 or 60006 
volts. 

Question 16. Into what classes may arresters be di= 
Vided as regards to the design of the arrester? 

Answer. (1) Single gap arresters where one place is 
provided for the lightning to jump across. 

Single gap arresters are often installed in banks in par^ 
allel* so that many places are provided at once, 

The old saw tooth arrester is really a bank of singlfe 
gaps. 

(2) Multigap arresters, in which we have a 
number of single gaps in series. 

These are sometimes simply a set of arresters in series. 
Each arrester being designed for say 2500 volts, using 
four in a series will protect a 10000 volt circuit. 



*The words series, parallel, and shunt will be more fully 
explained in Lesson 18, but it will be sufficient now to state 
that if a current goes through all of a number of instruments or 
resistances, they are in series. If the current splits and part 
goes through one set of instruments or resistances and the 
rest goes through another set then these two sets are in par- 
allel. 

Each set while in parallel with the other set may of course 
consist of several pieces of apparatus in series. 

When a circuit is cut and a new piece of apparatus is inserted, 
this piece is in series with the other. 

When a circuit has a new piece of apparatus attached by 
soldering on the wires without cutting the original circuit the 
new piece is a shunt circuit and the part of the old circuit is 
said to be shunted. 



102 



ELEMENTARY ELECTRICITY 



Usually they are so designed that a single arrester con- 
sists of a set of gaps in series. These arresters can be 
placed in series for high voltage. 



Serves Gxrcurt 
\oo AxayeteV\ \ o c 




\oo 



Parallel Cxrcvxvts 



\oo Amperes 




S\\ort GvcaxvK 
90 




T\xe ^ art o£ t\xe mam Ivae \nU\c\x carrVes 
^o aacp.xs sYuit&edu b\}fc\x& s\uxr^ \M\u.o\i 
carrres \0 amperes. 

(3) Arresters with series resistances. The 
idea being that lightning will pass through the resistance 
without being obstructed much while the normal line 
pressure cannot send enough current through the resist- 
ance to hold an arc between the discharge points. 



LIGHTING ARRESTERS — LOW VOLTAGE 



103 



(4) Shunted resistances. In this type a re- 
sistance is put in parallel with the spark gaps. Experi- 
ment has shown that by proper design this is very effect- 
ive in preventing an arc across the discharge points. 

(5) Fixed gap length. In some arresters the 
gap length is fixed, and resistance (series or shunt), also, 
and the kind of metal used for the points, is used to sup- 
press the arc. 

(6) Lengthened gaps. The gap points are 
shaped like horns and the heat of the arc lengthens it by 
the uprush of hot air or the arc is forced up by magnet- 
ism. 




Fig. 34. Lightning Arrester and Stray Current Protector for Telephone, 
Bell and Signalling Circuits. 



In other types the gap points are drawn apart by mag- 
netism. 

Question 17. What type of arrester is used for tele- 
phone circuits ? 

Answer. The protector is shown in Figs. 34 and 35. 
The line current enters at binding post A and passing 
along the spring B goes through the pin P through the 
wire of the coil SC on to post E, where the instrument 
wire is attached. 

Each side of the apparatus is just alike, there being one 
piece for each line wire. 

On the left of post A are two carbon blocks, C and Cj, 



104 



Elementary electricitV 



separated by a slip of mica M with a circular hole in it. 
The upper carbon block has a drop of fusible metal let 
into its lower face, but it is flush with the carbon. 

The upper block is in contact with post A by a spring 
Which holds it in position. The lower block rests on a 
metal plate* which is connected to the ground wire D. 

When lightning or any pressure over 300 volts comes 
on the line it jumps across the air gap between the carbon 
blocks (whose length is equal to the thickness of the mica 
strip) and goes to ground. It at the same time melts the 
drop of metal, making a complete ground. The instru- 
ments are then absolutely short circuited and protected. 




Fig. 35. Top view of Arrester shown in Fig. 34. 



This means that there is a short and low resistance path 
for the current, which lightning will follow instead of go- 
ing through the instruments. 

Should there be a cross connection with other lines or a 
leak to the telephone line, the instruments could be dam- 
aged by the amount of current, while the pressure was far 
below 300 volts. 

In this case the "sneak current," as it is called, goes 
from A along the german silver spring B, up through P 
and through the sneak coil SC. 



LIGHTNING ARRESTERS — LOW VOLTAGE 



105 



The sneak coil is of very fine german silver wire, about 
30 ohms resistance and in a few seconds this coil gener- 
ates enough heat to melt a plug of fusible metal which 
holds the pin P in place. 

The spring B then moves up and touches the ground 
strip G, thus grounding the line and protecting the in- 
struments. 




Fig. 36. Lightning Arrester with Magnetic Blow-out 



Question 18. What type of arrester is used on moder- 
ate voltage lines ? 

Answer. One type is shown in Fig. 36. The air gap is 
between the curved plates. The magnet below is excited 
from the dynamo and the arc when formed is blown up- 
wards until the space at the upper end of the curved 
plates is too long for the pressure to maintain the arc. 



106 



ELEMENTARY ELECTRICITY 



The instrument acts as if the arc were blown out by a 
puff of wind. 

Another form of this arrester has two flat plates so sur- 
rounded by the magnetism that the blow out effect is 
stronger, and it is relied on, there being no horns to help. 
Both of these are used on direct current circuits. 

Question 19. Describe an arrester for alternating cur- 
rents at moderate voltage. 

Answer. There is a non-arcing arrester for A. C* 
work. It consists of seven cylinders., each one inch in di- 
ameter and three inches high. They are made of white 







Fig. 37. 



Wurtz Non-arcing Lightning Arrester. 
Alternating Current. 



Used with 



brass with a large percentage of zinc, and very little cop- 
per, in it. They are knurled or checkered so that the sur- 
face is covered with little points. 

These cylinders are held in insulating strips so as to be 
about 1-64 of an inch apart. For low voltages the center 



^Abbreviation for alternating current. 



LIGHTNING ARRESTERS LOW VOLTAGE 



107 



cylinder is grounded and the end ones connected to the 
lines. 

When used on A C circuits the discharges which spark 
across do not cause arcs. 

The probable reason is that the cylinders being close 
together, the spark makes a little explosion which blows 



1000 VOLTS 




LINE LINE 

Fig. 38. Wiring diagram for 1000 volt circuits. One Arrester used. 



the arc out, and the boiling of the metal where the spark 
jumps carries the heat away in the vapor and the spot is 
too cool to hold an arc. 

The cylinders must be turned after each storm to pre- 
sent fresh surfaces for the next discharge. 

A single arrester is shown in Figs. 37 and 38. 
Figs. 39 and 40 show the arrangements for higher volt- 
ages. 

It will be noticed that this is of the multigap type. 



108 



ELEMENTARY ELECTRICITY 



[In Fig. 40 is shown the beginning of the "new idea" 
in lightning arresters which will be discussed at length 
further on.] 

Question 20. How does the non-arcing arrester in Fig. 
40 operate? 



2000 VOLTS 




GENERATOR 



W 



£t 



W 



GR OUND 

LINE LINE 

Fig. 39. Wiring diagram for 2000 volt circuits. Two Arresters used. 



Answer. The operation of this arrester is as follows : 
The number of series gaps is ad justed to the voltage at 
which the arrester is desired to discharge. This is the 
real lightning discharger. The series resistance is small 
and so wound that it is as little like a coil in its choking 
action as possible. Its presence will prevent a large cur- 
rent flowing through the arrester while it is discharging. 

If only as few series gaps as are shown were there, with 
a small series resistance, the dynamo current which fol- 



lows the lightning discharge will cause an arc and burn 
the cylinders. 
When shunted gaps are used the result is: 



LIGHTNING ARRESTERS — LOW VOLTAGE 



109 




The lightning followed by the line current passes 
through the series gaps. Then the lightning due to the 
choking action of the shunt resistance, sparks through the 



110 ELEMENTARY ELECTRICITY 

shunted gaps, while the line current on account of the 
high resistance of the shunted gaps, passes through the 
shunt resistance. 

There is then no line current in the shunted gaps to 
hold an arc. The lightning having now discharged the 
line current finds a series circuit, composed of the series 
gaps, the shunt resistance and the series resistance. 

The shunt resistance being large, the total resistance of 
the arrester is large enough to shut off the line current en- 
tirelyc 

Had such a large resistance been in series with the 
series gaps at first the arrester would not have started to 
discharge and of course afforded no protection. 

Question 21. Is there a non-arcing direct current ar- 
rester ? 

Answer. Yes, the non arcing direct current arrester is 
based on these facts. 

(1) Lightning will pass over a non-conducting sur- 
face more readily than across an equal air gap. 

(2) It will pass even more readily if the surface is 
covered with carbon. 

(3) An arc cannot form where there is no air to help 
the material burn. 

A lignum-vitse block is charred in its center for about 
half an inch in width. Two metal plates are set flush in 
the block on each side of the charred strip. 

A second block is screwed tightly over the first to keep 
out the air. 

This arrester works on direct current up to 700 volts. 
The lightning passes easily from plate to plate ; while the 
charred strip cf about 50000 ohms resistance prevents the 
passage of current from the line. 

The lightning cannot start an arc in this small space. 



LIGHTNING ARRESTERS — LOW VOLTAGE 111 

One plate is connected to a line wire and the other to 
the ground. Two should be used, one on positive wire 
and the other on the negative. 

These arresters have been used with "smooth cored" 
alternating current generators furnishing iooo volts pres- 
sure. 

Question 22. Is there a small, cheap arrester for single 
instruments and small buildings, as switch men's cabins, 
tool houses, etc., and for use on electric light circuits ? 

Answer. Yes. A lightning arrester designed for alter- 
nating current (abbreviated A. C.) up to 350 volts pres- 
sure is shown in Fig. 41. 

Where long secondaries are run from transformers, a 
necessity has been found for the use of lightning arrest- 
ers. The demand for a low priced but effective and reli- 
able arrester for this service has resulted in the arrester 
shown. 

This device is for use on any A. C. circuit of 350 volts 
or less, and is suitable for protection of individual series 
A. C. arc lamps, as well as on incandescent lighting cir- 
cuits. Its effectiveness when placed on wires at the en- 
trance to buildings, store-houses, signal towers, etc., rec- 
ommends its general adoption. 

A detailed description of construction is given later. 
The general plan is shown in Fig. 41, where will be seen 
the two large circular discharge plates separated by an 
air-space of 1/50 (.020) inch at their beaded edges. Over 
this air-gap a heavy discharge may pass, while light dis- 
charges and static surges will pass to earth, more slowly, 
through the high resistance disc that separates the larger 
metallic discs. This disc is of permanent resistance and 
allows the passage of but an infinitesimal normal current, 
while permitting the escape of the high voltage static dis- 



112 



ELEMENTARY ELECTRICITY 



charges. In the event of the discharge being heavy, it 
will jump the spark gap, but the low voltage of the nor- 
mal current will not maintain an arc, owing to the cooling 
effect of the heavily beaded disc. 





Diagram of connections 

dARTON-DANWS-lKHTNlNOAmSi 
TYTI T.N°300 
-10R- 

Skondariis or transformers 

VT TO 350 VOLTS. A.C. 
GARTON-DANLELS company 
. IOWA. U.S.A. 



Fig. 41. Small Arrester 
for Alternating Current 

up to 350 volts. 

Fixed gap type. 



It will be seen that this device offers a choice of either 
of two paths, one highly efficient as an outlet for static 
surges, and the other (the spark-gap of 1/50 inch) a 
highly efficient path for lightning discharges. 

When used on the secondaries of transformers, one ar- 
rester is necessary on each leg of the circuit. Same should 
be connected in a shunt path to earth as shown in Fig. 42. 

As an arc lamp protector it is connected directly across 



Lightning arresters — low voltage ll3 

the terminals of the lamp as shown in Fig. 43, thus offer- 
ing a path around the lamp, to the standard pole 
arresters, which should be distributed along the line at 
intervals. These standard {)ole arresters are, of course, 
connected between line and ground, and thus offer an easy 
escape for the discharges. 



TYRE T. 300 

CiamhhmuljeifmiKjUtUfTBt 




Diagram °f Connections 

GA RTON —DAM I ELS 
UCHTSISG ARRESTERS 

TYPE T. N? 30O 

EOK ALT. CURRENT SERIES 

ARC LAMPS 

UP TO 3SO VOLTS 

CARTON-DANIELS COMPANY. 

KEOKUKKWA.UJJL 

Fig. 43. 



It has been customary in many cases to use standard 
forms of arresters in the same service for which this de- 
vice is designed. These standard forms have the objec- 
tion of higher cost, larger size, and, as all employ a 
much greater spark-gap distance, are not nearly so effi- 
cient as this Type T. arrester. Furthermore, the auxiliary- 
path through the high resistance disc increases the effi- 
ciency of this arrester many times. 



114 



ELEMENTARY ELECTRICITY 



The device consists of the parts illustrated in Fig: 44 
assembled as shown in side view Fig. 41. Parts Nos. 309 
are two metallic discs, formed with a heavy bead around 
the circumference, the center being flat to make contact 
with the high resistance disc, No. 311. These parts are 






309 309 

OO 




3H 

O 



312 



3g8 313 

OO 



310 



310 



Fig. 44a. Parts of Arrester shown in Fig 41. 



assembled on the insulating tube, 312. The high resist- 
ance disc separates the metallic discs so that the heavy 
beaded circles are separated by 1/50 (.020) inch. Parts 
Nos. 308 are insulating discs, also mounted on 312. The 
screw, No. 306, passes through steel washer, 307, tube, 
312, and asbestos disc, 313, so as to clamp them together 
when screwed into weather-proof box, 302. The flexible 
leads, No. 310, pass through porcelain insulator, No. 



LIGHTNING ARRESTERS — LOW VOLTAGE 115 

303. The cover, 301, hooks over the top of box, 302, and 
is fastened in place by but one screw, 305. This cover is 
perfectly weather-proof. Complete, the arrester measures 
2^/2 inches from center to center of supporting holes. 




Fig. 44b. Arrester parts as shown in 44a assembled ready to 
screw cover on. 



Protection of Lines. 

Question 23. How should signal lines be protected? 

Answer. When on pole lines an arrester should be 
placed on pole about every half mile ; when run in con- 
iuits or tunnels one should be placed at each end of con- 
luit or tunnel. 

Question 24. How should telegraph lines be pro- 
ected ? 

Answer. As the instruments are only placed in sta- 
ions, an arrester at point where wires enter station is 
ufficient. 

Question 25. How should telephone lines be pro- 
ected ? 

Answer. A protector which contains a "sneak cur- 
^ent" device should be placed in every line where it enters 
1 building. 



116 ELEMENTARY ELECTRICITY 

Question 26. How should feeders to trolley wire or 
third rail be protected? 

■Answer. Such lines are usually fairly short and not 
much exposed to lightning being carried on low poles; 
fhe arrester on the 1 feeder at the station and one where 
the feeder connects to trolley or third rail should giv6 
ample protection. 

Question 27, How should trolley wires or third rails 
be protected? 

Answer. The arresters at the ends of the feeders 
ought to be sufficient for the trolley wire also. 

If there are very few feeders it would be well to see 
that there is an arrester every half or three quarters of a 
mile. One to the mile will do, as trolley construction is 
very strong and not liable to damage. 

The third rail lies along the ground and is seldom 
struck. If it were the mechanical strength of the rail it- 
self and its insulators would protect them, although the 
lightning did side flash to the ground. 

Question 28. What protection should motor cars or 
locomotives have? 

Answer. There should be an arrester in the main cir- 
cuits which furnish the power, and an arrester in the 
control circuits which control the motors. These two 
arresters should be of different styles. That for power 
circuits should discharge at 1,000 volts and that in the 
control circuits at 250 volts. 

Question 29. What protection should be given to a 
trolley car ? 

Answer. An arrester should be placed on roof or un- 
der hood of such a capacity that there can be no chance 
of its failing to operate. 



•e 



LESSON 8. 

Lightning Arresters. 

high voltage. 



An arrester of the two gap type with a magnetically 
lengthened gap to break arc is shown in Fig. 45, and its 
construction in Fig. 46. It is made for alternating or 
direct current work. 

In order to increase the surface distance, so as to pre- 
vent breakdown between current carrying parts of con- 
siderable difference of potential, one of the discharge 
points is mounted on the end of the resistance rod B. 
This rod is held in position by the clamps at C and D. 
The distance from clamp C to upper discharge point A is 
2^4 inches. The solenoid cut-out coil H is supported by 
brackets I and K, bracket K being so designed that it 
gives a surface distance on the porcelain base of 2^4 
inches between K and lower discharge point bracket L. 
This is a total of 5 inches, which is a liberally safe sur- 
face distance on porcelain for 2500 volts. 

To still further reduce the possibility of current jump- 
ing between parts, the line connection is at the top of the 
Arrester, from which the discharge passes downward in a 
practically straight path to ground connection. This path 
is indicated by the round dots in Fig. 46, the dashes show- 
ing the path of the normal current. It will be noted that 
the discharge goes through the section of the resistance 
rod C-D, the normal current being shunted through the 

117 



118 



ELEMENTARY ELECTRICITY 



solenoid coil H. This energizes the iron armature J, 
which raises upward in the coil, opening the circuit be- 
tween the discharge point M and lower end of armature. 
The discharge point M is stationary, so that the air-gap 





Fig. 45. 2500 volt Alternating Fig. 46. Diagram of Arrester 

Current Arrester. Mechanically shown in Fig. 45. 

lengthened gap type. 



at N is not changed by the operation of the Arrester. The 
arc is thus drawn out until broken inside the tube, which 
is practically air tight and prevents flying sparks or seri- 
ous arcing. The upper end of discharge point M is car- 



LIGHTNING ARRESTERS HIGH VOLTAGE 119 

ton, so the arc is drawn out between the same and the 
iron armature. This combination prevents sticking or 
welding together. . 

3500 volt arresters are designed along the same lines as 
the 2500 type. They differ in width and length of base to 
accommodate the higher voltage rating per arrester unit. 
In the 3500 volt arresters is provided an additional air- 
gap near the line binding-post. This construction has 
proven in extended service to be perfectly safe, and satis- 
factory for the higher, voltage rating of 3500 volts. 

As the circuit is opened inside the tube and the air-gap 
adjustment is always the same, it is possible to use the 
small air-gap space. In this 2500 volt arrester, the air- 
gap distance is 3-32 inch, which is as small as can be used 
safely. 

The cut-out is entirely automatic, restores itself by 
gravity, is instantaneous in operation, and prevents 
grounding the line, whether the discharge points are dirty 
from repeated operation or not. If the normal current 
follows the lightning over the air-gaps, it is shunted 
■through the coil. The coil immediately cuts it off and the 
normal dielectric of the air-gap is restored. 

To limit the flow of normal current that can follow the 
discharge to ground, the upper section of the resistance 
rod B is employed, there being approximately 250 ohms 
between discharge point A and clamp C in the 2500 volt 
arrester. This keeps the current down to a value that is 
broken readily by the cut-out, and is not enough resist- 
ance to impede the passage of the discharge. 

This feature is particularly effective where a part of 
the circuit is grounded, or where the circuit is temporarily 
or accidentally grounded. This series resistance prevents 
a heavy short-circuit through the arrester. The cut-out 



120 



ELEMENTARY ELECTRICITY 




Fig. 47. 10 000 volt Arrangement of Four 3000 volt Arresters. 



LIGHTNING ARRESTERS HIGH VOLTAGE 121 

readily interrupts the flow of normal current, and the ar- 
rester is again ready for another discharge. 

The positive action of the cut-out renders the arrester 
independent of the condition of the discharge points, and 
they require no more than an occasional inspection. 

When protection for higher pressures is desired a num- 
ber of these arresters are often mounted in series as 
shown in Fig. 47. 

The multigap arrester with graded shunt resistance for 
alternating current. 

If a series of knurled cylinders of zinc alloy about 1/64 
of an inch apart has the first one connected to a line and 
the last one grounded, there will be an electric strain 
all along the series due to the tendency of the pressure 
on the line to force electricity through the series to the 
earth. 

Suppose a single gap between two cylinders will al- 
ways prevent 400 to 600 volts from jumping across, but 
that 800 volts will be sure to jump. 

Suppose you have a 22000 volt line and place 56 cyl- 
inders in line making 55 gaps, each gap will stand 400 
so the 22000 volts will never spark across. 

Suppose a lightning discharge increases the pressure 
on the line to 33000 volts, which makes 600 volts per 
gap. One would think that the gap will not be jumped* 
and that the insulation of the machines, etc., will be 
strained. The frequent occurrence of this will finally 
break down the insulation and a burnt out generator be 
the result. 

However, an arrester of a large number of gaps works 
in a peculiar manner. 

When the normal 22000 volts is on the arrester the 



122 ELEMENTARY ELECTRICITY 

first gap has a pressure on it of 600 to 700 volts and each 
successive gap less and less on to the end. 

The pressure does not distribute itself evenly over all 
the gap but piles up on the first few. 

It is clear then that when the abnormal 33000 comes 
on the line that the first few gaps of the arrester have a 
pressure of 900 to 1000 on them, and the spark jumps 
across. 

The state of affairs is now as if 52 gaps were placed 
as an arrester on a 31500 voltage. The first few gaps 
getting the highest pressure are sparked across. 

This action takes place all along the 55 gaps, each gap 
nearest the line being sparked across by the concentra- 
tion of pressure upon it, until all the 55 gaps are spark- 
ing. The discharge passes to the earth and the line is re- 
lieved. 

Another peculiar thing now happens. When all the 
gaps are sparking the voltage distributes itself evenly 
over the arrester; so that now only 600 volts are across 
each gap and the sparks go out before any of the current 
of the line can flow through and cause arcs. 

If line current should follow the lightning discharge 
this current by its action brings the arrester more quickly 
to the even distribution state, and the arcs go out. 

If the discharge did not completely relieve the line a 
second one would immediately occur, or a succession of 
them until the pressure was down to 22000. At that pres- 
sure there would be 600 to 700 volts on the first gap and 
very little on the last. 

It is evident that in designing a multigap arrester you 
cannot divide the line voltage by 400 and put in that many 
gaps in series and be positive that 22000 volts won't dis- 
charge through it under all conditions of regular use. 



LIGHTNING ARRESTERS HIGH VOLTAGE 123 

You must remember that extra gaps are needed as you 
increase the number in the arrester. Three gaps will hold 
1 200 volts while 120000 volts will go right through 300 
gaps. Ten times as many gaps will not hold back ten 
times the pressure. 

This arrangement of a large number of gaps called the 
multigap arrester has three excellent features. 

( 1 ) It will discharge at a very slight increase of pres- 
sure above the normal. 

(2) It automatically stops discharging when pressure 
on line falls to nearly the normal pressure. 

(3) Line current going through the arrester carries 
its own cure. 

This condition of high pressure existing at the end of 
a series of gaps may be illustrated by connecting ten in- 
candescent lamps (no volt type) suddenly to a 1000 volt 
circuit. There will then be a surge which will generally 
burn out a lamp or two at each end of the series. Those 
lamps at the center of series will never be damaged. 

Remember this is not a proof of the high pressure at 
the first gaps because the lamps are broken by a surge, 
but it does prove that pressures can pile up at a point in 
a circuit far above normal, and makes one willing to be- 
lieve that the gaps might do a similar thing. 

The objection to the multigap arrester is as peculiar as 
its action. 

A static discharge is of low frequency and a lightning 
stroke discharge of high frequency. 

A high frequency pressure causes a greater pressure on 
the first gap than a low frequency pressure. 

Hence, an arrester discharges at a lower pressure for 
lightning-stroke, than for static accumulations. 



124 ELEMENTARY ELECTRICITY 

If then enough cylinders are used so that the regular 
voltage on line won't break* down the gaps, there will be 
often static accumulations of higher voltages than is safe 
for line but of such low frequency that enough pressure 
will not be exerted on the first gap to start the arrester 
into action. 

It is the multigap arrester with graded shunt resist- 
ances that solves this problem. 

Low frequency pressures can only break down a few 
gaps as compared with a high frequency pressure of same 
voltage. 

rOOOOOOOOOOOOOOOOOOCh 

ooooooooooooo 
ooooooooo-^ 



4- 



LOW RESISTANCE 



MEDIUM RESISTANCE 



KDOOOCk 



HIGH RESISTANCE 

Fig. 48. Diagram of a Multigap Arrester in Imperfect Form. 

Suppose as in Fig. 48 there are arranged between line 
and ground four circuits, one of 18 gaps, another of 12 
gaps and a low resistance, a third of 8 gaps and a me- 
dium resistance, while the last has 4 gaps and a high re- 
sistance. 

It will be seen that the opposition offered by any of the 
four circuits to 6600 volts will be perfect and no line cur- 
rent will pass through the arrester. 

A lightning stroke of high frequency will pass through 

*The words "break down" used with lightning arresters do 
not mean any damage to apparatus but merely refer to the dis- 
charge, ' 



LIGHTNING ARRESTERS HIGH VOLTAGE 



125 



r I the 18 gaps quite easily. Surges will perhaps be unable 
to pass the 18 gaps, the frequency being too low, but will 
find their way through one of the other circuits. Static 
accumulations being of very low frequency will pass 
through the 4 gaps and high resistance while they could 
not get through any of the other three. Remember that 
any of these must be above the normal pressure to dis- 
charge. In fact, unless they were above normal voltage 
we would not care about them. 

The objection to this arrangement of gaps and resist- 
ances is that a static charge of very high pressure and 
very low frequency might occur. 

<- -OOOOOOOOOOOOOOOOOOOt 



LOW RESISTANCE 



MEDIUM RESISTANCE 



Fig. 49. 



HIGH RESISTANCE 

Perfected Form of Multigap Arrester with Graded Shunt 
Resistances. 



This could only break down the 4 gap high resistance 
leg* of the arrester on account of low frequency. It 
would discharge so slowly through the high resistance 
that the line would not be freed from the high pressure 
quickly enough to prevent damage. 

The arrangement of gaps and resistances actually used 
is shown in Fig. 49. The multigaps are put in series and 
the three resistances are put in as shunts. 

* Parts of circuits which have the same starting and ending 
points are often called legs. 



12G ELEMENTARY ELECTRICITY 

This removes the objection just mentioned and inci- 
dentally uses far less cylinders in an arrester. 

The action is just the same, each frequency selecting its 
own path. In addition to this each time the arrester acts 
no matter what the frequency, the whole line of gaps 
from end to end breaks down and relieves the line 
quickly. 

In fact, with this arrangement we might say that the 
resistances are merely a device to enable a low frequency 
to break down more gaps than it usually can. 

This action takes place as follows : When a low fre- 
quency discharge passes through the high resistance and 
sparks across the last 4 gaps ; at the same time, part of 
it passes through the medium resistance, and as the last 
4 gaps are sparking it is able to break through the 4 in 
front of it to them. This action occurs all the way up 
the arrester. 

The action can also be explained in this way : The low 
frequency pressure passes through the three resistances 
and exerts its pressure at different points along the gaps. 
At only one place does it find few enough gaps between 
itself and the ground. This place is the last 4 gaps. It 
breaks these down and begins to discharge to earth. But 
now at the end of the medium resistance connection it 
finds only 4 unbroken gaps, and is able to break them 
down and it does so. 

In this way the whole arrester breaks down in sections, 
even for low frequency discharges. 

In Fig. 50 is shown a 2300 volt arrester with two re- 
sistances and in Fig. 51 is shown a set of cylinders 
mounted on slate base. These sets of cylinders are used 
in building up the 6600 volt up to 60000 volt arresters. 



LIGHTNING ARRESTERS HIGH VOLTAGE 



127 




Fig. 50. 



A 2300 volt Multigap Shunt Resistance Arrester for 
Alternating Current. 



With these high voltages a long spark gap shunted 
by a fuse is placed in series with the arrester. Then on 
a heavy short circuit the fuse blows and puts the spark 
gap in series with the arrester. 



128 



ELEMENTARY ELECTRICITY 



This prevents the destruction of the arrester and does 
not put it completely out of action. The fuse should be 
replaced as soon as possible. Frequent inspections should 
be made to see that cylinders are clean and the fuse in 
working order. 




Fig. 51. Set of Cylinders on Slate Base. These are used as units 
to make up complete arresters. 



r 



PROTECTION OF LINES. 



Transmission Lines, 



Question i. What is one of the simplest methods of 
protecting an overhead line ? 

Answer. Using a ground wire. 

Question 2. What is a ground wire ? 

Answer. It is one or more wires strung parallel to the 
line and grounded at every pole. One is about as good as 
more and very much cheaper. 



LIGHTNING ARRESTERS — HIGH VOLTAGE 129 

Question 3. In what position are they placed? 

Answer. When there are two line wires one ground 
wire should be run on the top of the pole, or two ground 
wires run; one at each end of the cross arm or another 
cross arm below the main arm. 

When there are three line wires, one ground wire 
should run on top of pole or in the center of the triangle 
formed by the line wires, or three ground wires should 
be installed, one on top of the pole and one at each end of 
a cross arm under the main cross arm. 

Question 4. Why are they run on glass insulators? 

Answer. They must be tied to something and a cheap 
glass insulator is less expensive than some special device 
which is not an insulator. The fact that the glass is an 
insulator is not harmful, because a ground connection is 

Imade by a wire at every pole. 
Question 5. Are the ground wires copper? 
Answer. They are usually galvanized iron about No. 
4 size but a stranded $4 "cable" is better. 

Question 6. Is the iron wire as good as copper for a 
ground wire? 

Answer. Yes, for electrostatic charges the size of the 
wire is of far greater importance than the material. 

Question 7. Barbed wire is generally used, is it not? 

Answer. Formerly it was, but engineers are now be- 
lieving that the plain wire is as good, and being cheaper 
and much easier to handle is much more used than the 
barbed. 

Question 8. What was the reason for originally using 
barbed wire ? 

Answer. It was thought that the barbs acted as dis- 
charge points to let the free charges escape quickly into 
the air. 



130 ELEMENTARY ELECTRICITY 

Question 9. Do they not act that way? 

Answer. They probably would if the free charge was 
not neutralized by the earth connection so quickly. 

A plain grounded wire well connected to earth is suf- 
ficient protection. 

Barbed wire is not as strong as a plain wire of equal 
weight per foot. 

Question 10. Why are not ground wires always put 
above the line wire? 

Answer. They used to be put above, because engi- 
neers thought the ground wire was a protection from a 
direct lightning stroke. 

We now think that they are not much use in that case, 
so do not always put them above so as to have them 
struck first. 

We often put them below believing that, in the way 
that they protect, they can do so as well from there as 
from above. 

Furthermore, being below should they break they can- 
not cause short circuits by falling on the line wires. 

A single ground wire is often put on the top of the pole 
for convenience, but a single ground wire with three 
line wires is sometimes put in the center so as to be 
equally distant from all three. 

Question II. How does a ground wire protect the 
line ? 

Answer. It seems to be a fact that lines are not often 
struck by lightning, but that thunder or electrical storms 
affect the lines by static charges very frequently. 

This is done as follows : A cloud heavily charged with 
say positive electricity blows up over the line. There 
will be induced in the line a bound negative charge and 



LIGHTNING ARRESTERS — HIGH VOLTAGE 131 

a free positive charge. This free charge will have a 
tendency to go to the earth. It may do so by leakage 
over and through the insulators of the line if the approacn 
of the cloud is slow enough to allow it to do so^ if not it 
jumps through the insulator puncturing it, or it may 
side flash over the insulators from wire to cross arm. 

If a ground wire is present a bound negative and a 
free positive charge is induced in it. 




++++++ 



V\T\.e Vfvce 

p+Tfc ^ t + + + G 

Metvdetvc^o TetAucv^G^uTvc-- 

Fig. 52. Electrostatic Charges on a Line under a Thunder Cloud 
Without a Ground Wire. 



This bound negative charge prevents as great an elec- 
trical separation on the line as the cloud alone would 
make and so the line does not become so highly charged. 

This is shown in Figs. 52 and 53. 

In Fig. 52 suppose the cloud to cause an electrical sep- 
aration in the line as shown. There will be a tendency 
to puncture or side flash at points A and C. 

Now suppose the charge on the cloud to be neutralized 
by a lightning flash from cloud to earth. If it does not 



132 ELEMENTARY ELECTRICITY 

strike the line at B and neutralize the negative charge 
there, it will leave this charge free and there will be a 
tendency to puncture or side flash at B. After this the 
charge at A and B will spread over the line with a surge. 



+ + + + 




\_u\.e 



+ + 



+ 4- _f- Yf\xe 



H 



G\:o\m<X 









Wire 



Earttv 



Fig. 53. Electrostatic Charges on a Line having a Ground Wire. 



When the ground wire is there as in Fig. 53 the cloud 
induces a negative charge on the ground wire which is 
smaller than that on the line. It is smaller because the 
capacity of the ground wire is less than that of the line 
wires. It would be far too expensive to make the ground 
wire capacity nearly equal to the line itself. 

The free positive charge on the ground wire goes to 
earth and the bound negative charge acts inductively on 
the free positive charge of the line. This causes the dis- 



LiGHTNIXG ARRESTERS — HIGH VOLTAGE 133 

tribut'on of charge to be as is shown and the tendency 
to have static troubles at E and G is less than it would be 
without the ground wire. 

Suppose now the cloud is discharged. The charge at 
F oh line will not act as violently as that at B did, because 
the repelling effect of the negative charge on the ground 
line at F tends to spread out the negative charge on the 
line at F. Thus the tendency to static discharge at F is 
less than if ground wire were not there. 

Question 12, What are the objections to a ground 
wire? 

Answer. It is expensive to install. It is only a partial 
protection and other devices must be used with it. 

Question 13. What is its chief value? 

Answer. It prevents the splitting of the poles, and 
damaging of insulators. 

Question 14. Does it protect the station? 

Answer. No, it is a line protection. 

Question 15. What other simple protectors are there? 

Answer. The horn arrester is an extremely simple 
device. 

Question 16. What is a horn arrester. 

Answer. A horn arrester is as shown in Fig. 54. A 
single spark gap whose length is regulated by the pres- 
sure it is designed to withstand (say 6% inches for 90000 . 
volts) has a wide spreading pair of horns attached, the 
ends being perhaps twelve feet apart. 

Question 17. How are they attached to line? 

Answer. One side is attached to a line wire and the 
other side of gap is grounded. A fuse is placed in the 
ground wire. Each line wire has its own horn. 

Question 18. How do they work? 



134 



ELEMENTARY ELECTRICITY 



>d 



-^\vt\oj^ 







Lightning arresters— high voltage 133 

Answer. The discharge jumps across the gap form- 
ing an arc. The heat of the arc causes it to rise and as 
its ends are in the horns it is stretched out long and thin. 
This cools the arc down and it goes out. While the arc 
holds the charge on the line runs across it to the earth. 

If the arc does not go out the normal current on the line 
flows across the arc and blows the fuse. The fuse is 
made very long so that an arc cannot jump across be- 
tween the terminals which hold it. 

Question 19. What are the advantages of the horn 
arrester ? 

Answer. Fairly cheap. They cannot get out of ad- 
justment and discharge at wrong pressure. They cannot 
produce an accidental ground by getting out of repair 
or through defects in manufacture. They are mechani- 
cally strong and will stand the most severe strokes. 

Question 20. What are the objections? 

Answer. Any time they discharge the line the fuse 
may blow. The arrester is then useless until fuse is re- 
placed. 

The discharge is a vicious arc which sends a surge 
through line. 

Some engineers think the surge is worse than the origi- 
nal trouble. 

Question 21. What is the general opinion about them ? 

Answer. That they are an excellent thing to use at 
points on the circuit especially exposed to lightning. 

Question 22. What are single or side horns? 

Answer. As shown in Fig. 55, the ground wire of 
each pole is extended up beyond the top insulator and a 
branch run up outside of each side insulator. 

This construction is to protect the insulators by allow- 



13(3 



ELEMENTARY ELECTRICITY 






W*H£ 






TO TMIQ f»OINT TO 

fxccuoc vwKTe:* 

>WO TO^Uf>r>Q&T 
Gf*OUND W/ff£*. 



CLAM'* ron utei-rr***** *&° 




to Gf^outoo 



Fig. 55. Side Horns. 



ing the discharge to jump to the ground wire instead of 
flashing around insulator to the cross arm. 

This Fig. does not show a ground wire strung from 
pole to pole, 



LIGHTNING ARRESTERS HIGH VOLTAGE 137 

But horn arresters and side horns can be installed 
whether there is a line ground wire or not. 

Side horns are installed at every pole. 

Question 23. What is the lowest voltage used on 
transmission lines? 

Answer. 22000 volts, because at lower voltages the 
wires have to be so large that expense is too great. 



Mains, Feeders. 

Question 24. What is the highest voltage used on 
mains and feeders ? 

Answer. 11000 volts. Any voltage higher than this 
needs such special protection, that it should be run on a 
high pole line, off to one side of the right of way. 

The mains from power house or the feeders from sub- 
stations to the third rail or trolley wire can be run at 11,- 
000 with safety. 

Question 25. How should mains and feeders be pro- 
tected ? 

Answer. When on pole lines an arrester every half 
mile is the best practice. When underground, one at each 
end of the section. 

Question 26. Are choke coils used with these line ar- 
resters ? 

Answer. No. Choke coils are only used with station 
arresters. 

Unless an arrester is protecting machinery or instru- 
ments no choke coil is used. 



LESSON 9. 
Lightning Arresters, 
auxiliary apparatus. 

Question I. What is a choke coil? 

Answer. It is a coil specially designed to insert in a 
line between the apparatus to be protected and the light- 
ning arrester. 

Question 2. How are they made? 

Answer. For low voltage they are simple coils of in- 
sulated wire mounted on a slate bases as shown in Fig. 

56. 



mhii h' iiilm 

Fig. 56. Low Voltage Choke Coil. 



For high voltage (over 600) the wire is bare and the 
turns are wound in an hour glass fashion, so that air 
forms the insulation between turns. 

Question 3. How do choke coils act? 

Answer. A surge or stroke encountering a choke, re- 
active, or kicking coil in its path is momentarily held 
back,* practically stopped. The coil then begins to con- 
duct the charge. It will be seen that the choke coil offers 



*Throttled, choked off, kicked back, are some of the terms 
used. 

138 



ARRESTERS — AUXILIARY APPARATUS 139 

protection for an instant only, but during this time the 
arrester on the line side of the coil can free the line of 
the charge. 

The damming up of the surge by the choke coil pro- 
duces an enormous pressure which helps to force the 
charge through the arrester. 

Question 4. What is the objection to a choke coil? 

Answer. It has resistance, and so wastes energy, it 
will also retard a little the flow of the normal current. In 
order to prevent their interference with normal operation 
they are large and expensive. 




Fig. 57. Hour glass Type of High Voltage Choke Coil. 

Question 5. What are the advantages? 

Answer. They increase the protection offered by the 
arrester and even if arrester fails to act the choke coil 
protects to some extent by causing a side flash on the 
line where damage is less expensive than should it occur 
in the station. 

Question 6. How are lightning arresters installed ? 

Answer. They are placed in between line and ground 
and the two arresters which are attached to the line wires 
at a certain point are all connected to the same ground 



140 ELEMENTARY ELECTRICITY 

wire, so that there may be a free discharge between line 
and line, as well as between line and ground. 

Question J. What precautions must be taken when in- 
stalling arresters in buildings? 

Answer. The National Electric Code gives the fol- 
lowing rules for the construction and installation of ar- 
resters in buildings : 

1. Lightning arresters must be mounted on non-com- 
bustible bases, and must be so constructed as not to main- 
tain an arc after discharge has passed, and must have no 
moving parts. 

[The arrester shown in Fig. 45 has been tested and 
approved by the National Board of Fire Underwriters 
although it has a moving part.] 

2. Must be attached to each side of every overhead 
circuit connected with the station. 

It is recommended to all electric light and power com- 
panies that arresters be connected at intervals over sys- 
tems in such numbers and so located as to prevent ordi- 
nary discharges entering (over the wires) buildings con- 
nected to the lines. 

3. Must be located in readily accessible places away 
from combustible materials, and as near as practicable to 
the point where the wires enter the building. 

Station arresters should generally be placed in plain 
sight on the switch-board. 

In all cases, kinks, coils, and sharp bends in the wires 
between the arresters and the outdoor lines must be 
avoided as far as possible. 

4. Must be connected with a thoroughly good and 
permanent ground connection by metallic strips or wires 
having a conductivity not less than that of a No. 6 B. & S. 



ARRESTERS — AUXILIARY APPARATUS 1 11 

copper wire, which must be run as nearly in a straight 
line as possible from the arresters to the earth connec- 
tion. 

Ground wires for lightning arresters must not be at- 
tached to gas-pipes within the buildings. 

It is often desirable to introduce a choke coil in circuit 
between the arresters and the dynamo. In no case should 
the ground wire from a lightning arrester be put into 
iron pipes, as these would tend to impede the discharge. 

Question 8. How should arresters be grounded? 

Answer. For station arresters there are many ways of 
getting a good ground. 

Copper sheets approximately l /% inch thick and of 4 to 
6 square feet surface are suitable. A piece of cast iron 
of large surface, with brass or copper plug tapped into it 
for connections, is preferable. Cast iron does not waste 
away as rapidly, and, when completely oxidized, still af- 
fords a good ground path. 

The ground wire must be carefully riveted or soldered 
to the plate and the connection coated with a preserva- 
tive paint. 

The plate should be buried deep enough to be in damp 
soil the year through. The bottom of the hole should be 
covered with broken charcoal, coke or carbon to a depth 
of 2 or 3 inches. After the plate is put in position, it 
should be covered with another layer of charcoal, coke or 
carbon, and the hole filled with earth. It is well to use 
running water to settle. 

Ground connections may be made to plates placed in 
the mud at the edge of a stream. Where water or gas 
pipes are available, the ground wires should be soldered 
to a brass plug screwed into the pipe in addition to the 



142 



ELEMENTARY ELECTRICITY 



connection with the ground plate provided. In ground- 
ing arresters on electric railway circuits, a connection 
with the rails, as well as. with the ground plate, should 
always be employed. 

Where an iron pipe is used to protect the ground wire, 
the wire should be soldered to a cap on the upper end 
of pipe. This avoids the choking effect of the pipe upon 
a wire passing through it. 

For pole arresters a cheaper arrangement is necessary. 
The ground pipe is perhaps the best. 




Fig. 58. Ground Pipe Fittings. 



These may be driven into the earth, or if the soil will 
not permit driving, a hole may be dug at the foot of the 
pole to receive same. The pipe should extend upward 
along the pole for eight to twelve feet above the ground, 
to prevent cutting or removing the wire, and the ground 
wire soldered to a cap on the upper end of the pipe. The 
pipe should extend eight or ten feet below the surface 
where it will be in damp earth the year round. 

For this purpose are manufactured the fittings illus- 
trated in Fig. 58. These are tapped for use with ^4 inch 
iron pipe and consist of the brass cap GP 100, with lug 




ARRESTERS — AUXILIARY APPARATUS 



143 



for soldering in ground wire from the arrester. GP 101 
is a brass coupling for connecting upper and lower sec- 
tions. (It being more convenient to drive an 8 or 10 feet 
length and then couple on another length, than to drive 
a 16 or 20 feet length.) This brass coupling GP 101 is 
also provided with a lug for soldering in wire to rail, 
when used on electric railway circuits. The driving point 
GP 102 is of malleable iron, with dipped galvanized 
finish. 




Fig. 59. Ground Plate for Pole Arresters. 



The pipe may be driven by placing an iron cap on upper 
end, to protect the threads, or the method used by well- 
drivers may be used to advantage. This is to start the 
hole, fill with water and "churn" the hole to the required 
depth with the pipe. This method is of particular advan- 
tage where the soil is very hard. 

In Fig. 59 is shown a cast iron plate, 12 inches in di- 
ameter (total surface 450 sq. in.), with hole near edge 
tapped for 24 i nc h pip e - This plate may be used in 
place of the driving point and should be buried at the 
foot of pole, and the necessary length of pipe attached. 



144 



ELEMENTARY ELECTRICITY 



It may be buried at the bottom of pole before the pole is 
set, but if this is done, it should be made certain that the 
surrounding soil will be damp the year through. 

In electric railway circuits the rails should be connected 
to the ground wire as is shown in Fig. 60. 




0/AG&AAS SHOW/NG 

METHOD OFGROUNDING 
POLE ARRESTERS 

£LECTRIC RAILWAYS 
"0ARK)N:DAN1ELSC2- 

KEOKUK-, I A. 
U.S. 



Fig. 60. 



The rail alone will not suffice, as it may be up on a 
rock road-bed, or buried in cement or a soil that does not 
provide a low resistance path for the lightning. With a 
connection to both rail and ground point, any danger due 



ARRESTEES — AUXILIARY APPARATUS 



145 



to difference of static potential between rail and earth is 
avoided. Should either one of the connections be broken 
or fail from any cause, the other one probably will be in 
order and afford a degree of protection. 




Fig. 61. 

Fig, 61 shows manner of grounding a two wire or un- 
grounded circuit. 
All ground wires should be No. 6 gauge or larger. 



146 



ELEMENTARY ELECTRICITY 



Question 9. What inspection is necessary for light- 
ning arresters? 

Ansiver. Frequent inspection (once a month) and 
cleaning with a bellows for dust is necessary. 





Fig. 62. Low Voltage Arrester Disconnecting Switches. Single for 
Railroad work. Double for Electric Lighting. 



Question 10. Is there not danger in the men inspect- 
ing the arresters ? 

Answer. Disconnecting switches as shown in Figs. 62 
and 63 are installed in places where some other switch 
will not serve to disconnect the arresters. 



;-•-:' --.- 







Fig. 63. High Voltage Disconnecting Switch for Arrester. 






LESSON 10. 

Magnetism. 

introduction. 

The natural magnet has been known for ages. The 
Egyptians and Greeks in their writings long before the 
Christian era mentioned that a certain mineral attracted 
iron and steel. They also knew that a piece of steel be- 
ing rubbed or rather stroked with the mineral, acted 




Fig. 64. Natural Magnet : A piece of Magnetite or Black Oxide of Iron. 



just as if it had become a piece of the mineral. Fig. 64. 

Who first discovered that the mineral would point to 
the north, we do not know, but in the year 1200 A. D. 
Arabs brought compasses to Europe. 

The mineral was called lode stone because it was a 
leading stone i. e. it lead you to the north. 

The name magnet was also given to it because most of 
it came from a part of Greece called Magnesia. 

Besides the quantity in Greece large quantities of mag- 
netite are found in Sweden, Spain, also in states of Ar- 
kansas and New Jersey. 

147 



148 



ELEMENTARY ELECTRICITY 



Question I. What is a magnet? 

Answer. A magnet is a piece of material which will 
turn into a north and south position when suspended so 
as to be free to turn. 

Question 2. Can it be any material? 

Answer. No. It can only be of iron, steel or nickel, 
or a few other substances which are feebly magnetic. 

Question 3. But the mineral mentioned is a magnet? 

Answer. Yes, because it is iron ore. 

Question 4. Then copper, zinc, etc;, cannot be mag- 
netized? 

Answer. No. 






HI 



Fig. 65. Making a Magnet by use of a Permanent Magnet. 



U3 



Question 5. How can a magnet be made? 

Answer. Produce two pieces of Jessup's steel about 
6x%x^4 or 12x1x3-16 inches. Machinery steel, man- 
ganese steel, and some cast steels are useless. Heat 
them moderately bright red and plunge sidewise and 
edgewise into water or oil. They will become very hard 
and brittle, or as we say "glass hard." 

1. Lay one down on the table and stroke one-half of 
the bar from the center out to the end. Fig. 65. Do 
this ten times with the S-pole of a permanent magnet, 



MAGNETISM INTRODUCTION 149 

and turning the bar over repeat this on the same end. 
You now have an N-pole. Then using the N-pole of the 
magnet stroke the other end of the bar in the same man- 
ner. You now have the complete magnet with two 
strong poles. You have made 40 strokes in all and you 
need that many, but sitting there stroking for a 100 or 
more times is wasted energy, for the bar soon becomes 
saturated and will take up no more magnetism. Then treat 
the second piece in the same way. Always make two 
at the same time and keep them by laying them with an 
N and an S-pole at the same end of the box, separating 
the magnets lengthwise by a strip of card board, and 
placing a strip of tinned iron or strap iron across the 
ends. Laid away separately or with two N-poles side 
by side they lose strength. Fig. 66. 




Fig. 66. Two Bar Magnets with Keepers across ends. A represents 
north pole and B, south pole. 



As shown in Fig. 67 wrap insulated wire around the 
bar or on a spool in which the bar will be placed. Con- 
nect the wire to a battery, dynamo or electric light cir- 
cuit, and while the current is flowing in the coil tap the 
bar with a hammer. Stop tapping before the current, is 
turned off. No one unskilled in the handling of elec- 
trical machinery should do this, as one can cause con- 
siderable damage to himself and to the electrical wiring. 
Correct management of this process will produce the 
strongest possible magnet. 



150 



ELEMENTARY ELECTRICITY 



Question 6. Do magnets need to be handled care- 
fully? 

Answer. Yes. Dont: Heat, drop hammer, or file 
your magnet, for it will lose strength. 

Question 7. How should magnets be kept when not 
in use? 







Fig. 67. Making a Magnet by means of an Electro-magnet. 



Answer. Bar magnets should be laid side by side, 
separated by a thin strip of wood, the north end of one 
and south end of the other at same end. Strips of iron 
should be laid across the ends so as to touch both mag- 
nets as in Fig. 66. 

Horse-shoe magnets should have a strip of iron laid 
across the ends as in Fig. 68. 

Question 8. What is a magnetic needle ? 



MAGNETISM — INTRODUCTION 



151 



Answer. It is a long slender magnet, as compared 
with its own width and thickness, fitted with a cap in 
the center so that it may be balanced on a pivot or hung 
from a thread. This allows it to turn freely. Fig. 69. 




Fig. 68. A Horseshoe Magnet with its Keeper or Armature A. 



Question 9. How may you determine whether a bar 
is a magnet or not? 

Answer. To decide the question whether the bar in 
our hand is a magnet or not we put it to test in this way. 



152 ELEMENTARY ELECTRICITY 

Remember, however, that should the bar be of wood, 
fibre, copper, zinc, etc., there is no need of a test since 
only steel, iron and nickel have enough magnetism in 
them to be worth calling magnets. 



a-.. 




Fig. 69. Magnetic Needle. 



1. Bend a stirrup of copper or brass wire and sus- 
pend the bar in it. Hang the whole by a single thread 
that has been untwisted and soaked in water long 
enough to take out all the twist. If the bar persistently 
returns to the north and south line after being moved 
out of it; then it is a magnet. 

2. As a further precaution procure a magnet needle 
from some dealer and suspend it as in Test i. If it sat- 
isfies this test proceed to take your bar in hand and cau- 
tiously approach one end of the magnet. Should- it at- 



MAGNETISM INTRODUCTION 153 

tract it reverse the piece in your hand, and it should now 
repel. If it does all right, it is a magnet; if it does not, 
it is not a magnet. 

Question io. Why is the repulsion test more impor- 
tant than the attraction? 

Answer. Because any piece of iron or steel will be 
attracted to a magnet but only a magnet will ever be re- 
pelled. 

Question 1 1. Is this rule absolute? 

Answer. Hitherto only iron, steel and nickel have 
been mentioned as capable of being made magnets. Co- 
balt and manganese possess, limitedly, the same capabil- 
ity. Metals of this character are called Paramagnetic, 
and are attracted by the poles of magnets. There are 
other substances, among which are phosphorous, bis- 
muth, zinc and antimony, which act in a contrary man- 
ner, being repulsed by magnets. These substances are 
known as Diamagnetic substances. 

This repulsion is so weak that it can never be mis- 
taken for the repulsion of a magnet by a magnet. Fur- 
thermore, these diamagnetic metals are repelled by either 
end of a magnet. 

Question 12. What are the poles of a magnet? 

Answer. The ends of a magnet are called its poles. 

Question 13. How are the poles named? 

Answer. The end which points to the north geo- 
graphical pole is called the north pole of the magnet, 
the other end is called the south pole of the magnet. 

Question 14. How are the poles marked? 

Answer. The north pole with an N or a line cut in 
the steel, the other end is left unmarked. 

Question 15. What is the polarity of a magnet? 

Answer. By polarity we mean the nature of the mag- 



154 



ELEMENTARY ELECTRICITY 



netism at a particular point, whether it is north or south 
magnetism. 

Question 16. What are consequent poles? 

Answer. In long magnets extra poles may be found 
besides the poles at the ends. These extra poles always 
come in pairs. Such a magnet is shown in Fig. 70. 

Question 17. What rule gives the results of magnetic 
attraction and repulsion? 








Fig. 70. A Magnet with Consequent Poles. 



Answer. Either pole of a magnet attracts a magnet- 
izable metal. Like poles repel, unlike poles attract. 

If you take a magnet a foot long and brutally jab it at a 
tiny compass needle this rule may not work and the rea- 
son is this : The great power of the large magnet sweeps 
out the private magnetism of the compass and replaces 
it by magnetism whose poles are just the opposite to 
that which was formerly there. When the poles are re- 
versed, then where there was repulsion there will nov 
be attraction. 

Question 18. But the end of the magnet which points 
to the north geographical pole is caMed the north pole of 
the magnet? 



MAGNETISM INTRODUCTION 155 

Answer. Yes, for convenience we do say this and 
then to get out of the difficulty we say that the south 
magnetic pole of the earth is up at the north geographic 
pole. 

It must be definitely understood that when we speak 
of the "magnetic north pole" we mean that spot on the 
earth's surface which exhibits "south polarity." 

When we speak of the north pole of a magnet we can 
avoid any confusion by saying the "north seeking pole." 

Question 19. Has the earth polarity? 
■Answer. Yes, the region around the north geograph- 
ic pole has south pole magnetic polarity. Around the 
south geographic pole there is north pole magnetic po- 
larity. 

Question 20. Is not the north magnetic pole exactly 
at the north geographic pole? 

Answer. No. Standing in Chicago the compass 
points a little east of the geographic north. At San 
Francisco it points 16 degrees* east and in New York 
10 degrees west of the true north. 

Question 21. Why is this? 

Answer. The magnetic north pole is about 1400 miles 
south of the north pole and looking from New York 
about 10 degrees west of it. Why the magnetic pole 
should be here instead of at the north pole we do not 
know. 

Question 22. Are there any places where the compass 
points to the north pole? 



* If the circumference of any circle is divided into 360 equal 
portions, each is called a degree. A right angle embraces 90° 
( ° is abbreviation for degree). 

All degrees are not of the same size unless the circles happen tQ 
be the same size. 



156 ELEMENTARY ELECTRICITY 

Answer. Yes. There is a ring around the earth 
where the compass points north. This ring is an irreg- 
ular line. In the United States, Charleston, S. C, the 
east end of Tennessee, Columbus, Ohio, and Lansing, 
Mich., are on this Agonic line. It crosses Russia, Per- 
sia and Australia. 




Fig. 71. A Compass marked with Degrees as well as the Points of the 

Compass. 

Question 23. Is the magnetic north pole fixed? 

Anszver. No. In 1580 in London the needle pointed 
ii° east of north, it gradually swung over till in 1657 it 
pointed true north, it kept on till in 1816 it pointed 24 
west of north and then it swung back so that in 1907 it 
points about 15 west of north. It seems as if it took 
about 320 years to make a complete swing. 

Question 24. What name is given to this peculiarity 
pf the compass ? 



MAGNETISM — INTRODUCTION 157 

Answer. It is called the Declination of the compass. 
It may be easily remembered because the compass de- 
clines to point true north. 




Fig. 72. Dipping Needle. 



Question 25. What is a compass? 

Answer. The usual compass is a magnetic needle sus- 
pended over a card bearing the names of the points of 
the compass and sometimes the 360 of the circle are 
marked out. Fig. 71. 



155 ELEMENTARY ELECTRICITY 

Question 26. Why do home made compasses balance 
badly? One end seems too heavy. 

Answer. Xear the equator of the earth a needle may 
be balanced and then magnetized, but in other places 
if the needle is balanced and then magnetized one end 
of the needle will dip. Fig. 69 shows this. If the needle 
is mounted so that it can turn with perfect freedom ver- 
tically as in Fig. J2 it will in Chicago incline from the 
horizontal line about 70°. 




Fig. 73. Showing Dip or Inclination of Needle at Magnetic north pole, 
Magnetic equator, Magnetic south pole. 

Question 27. Is this Inclination of the needle the 
same all over the world? 

Answer. Xo. In Fig. 73 is shown the Dip or Inclina- 
tion of the needle at the magnetic north pole, magnetic 
equator and magnetic south pole. As you leave the 
equator going either way the dip increases until you 
reach the poles. 

Question 28. How is the dip neutralized in com- 
passes 

Answer. By balancing the needle after magnetizing, 
or if the compass must be used all over the world by at- 
taching a tiny sliding weight. 

Xorth of magnetic equator the weight is put on south 
end of needle and south of it it is put on the north 
end of needle. 



MAGNETISM — INTRODUCTION 159 

Question 29. What is the magnetic equator? 

Answer. It is an irregular line running around the 
world, being north of the geographic equator on Atlan- 
tic ocean and south of it on Pacific ocean. Along this 
line there is no dip to the needle. 

Question 30. Do you consider the earth a huge spher- 
ical magnet? 

Answer. Yes. These experiments tend to show that 
the earth is a magnet, for it magnetizes objects. 

Experiment 1. Procure a small pocket compass, and 
hold this so that the needle will move freely, against an 
iron stovepipe. Raise and lower it past the joints in the 
pipe, and as a rule the action of the needle will show 
that there is a change of polarity at each joint, the ends 
of the needle being alternately attracted in passing. 

Experiment 2. With the same compass explore the 
polarity of any permanent piece of iron, such as a bal- 
cony, an iron safe, a gas or water pipe which lies* in a 
north and south position, and it will generally be found 
that the north and south extremities between joints will 
show different polarities. 

Experiment 3. Explore the polarity of a street car 
rail lying in a north and south street. Its polarity will 
be found to be lengthwise of the rail. Now try a rail 
lying in an east and west position, and it will generally 
show a polarity at right angles to the length of the rail — 
the north side will show one polarity, while the south 
will show the other. 

Experiment 4. Take a fine cambric needle from a 
package which has been lying in a north and south po- 
sition, and drop it carefully on a glass of water. In the 
majority of cases, if properly handled, it will float, and 
generally show polarity by settling in a north and south 
position. 



160 ELEMENTARY ELECTRICITY 

Experiment 5. If now, while this needle is lying on 
the surface of the water we approach it carefully with 
the compass, one pole will be attracted, and the other 
pole will be repelled. That is, the two ends of the com- 
pass needle will repel the like ends of the floating needle, 
but will attract the dissimilar ends. This experiment may 
be made still easier by floating the needle with a tiny bit 
of cork through which it has been thrust. 

Question 31. Can the earth's magnetism be shown in 
any other way? 

Answer. Yes, if a bar of hard steel or even hard iron 
and some iron filings are procured. 

Hold the bar level in an east and west position and 
strike one end a smart blow with a hammer. Now dip 
it in the iron filings and we shall find it has little or no 
magnetism, at least in its length. Now point it down- 
ward at an angle corresponding to the latitude where 
you are, so as to point to the actual north magnetic pole 
as nearly as possible, and strike the end of the bar, as 
before. You will find that the bar has acquired a quite 
perceptible amount of magnetism; that either end will 
attract the iron filings, tacks or other bits of iron, and 
that the phenomena of attraction and repulsion will be 
shown by bringing it near the compass needle. 

Now, having marked the end which attracts the south 
end of the needle with paint or chalk as the N pole, we 
again point it to the north pole of the earth, but in a re- 
versed position, and strike it again as before. On test- 
ing for magnetism we will find that the particles of iron 
adhere as before, but what we marked as the N pole of 
our magnet has become the S pole, and repels the end of 
the needle it attracted before. 



LESSON ii. 
Magnetism — Continued. 

Question I. For the experiment in A. 31 of Lesson 
10 why is it necessary to take hard steel to get best 
results ? 

Answer. We know that wrought iron becomes a mag- 
net readily, steel castings are easily magnetized, cast iron 
less easily and hard steel is the most difficult of all to 
magnetize. The more difficult it is to magnetize a sub- 
stance, the better it retains its magnetism. If you want 
a permanent magnet use hard steel. If you want a tem- 
porary magnet use iron or steel castings. 

Question 2. Do you mean steel castings or cast steel? 

Answer. I mean steel castings which are made of 
Bessemer steel. Cast steel is a high grade steel which is 
expensive and is used for tools, etc. 

Question 3. Does magnetism permeate some metals 
better than others? 

Answer. Yes. We say that some metals have greater 
permeability than others. 

Wrought iron is the most permeable, cast iron the 
least permeable, of the cheaper metals. Hard steel has 
small permeability. 

Question 4. Do some metals retain magnetism better 
than others? 

Answer. Yes. The retentivity of wrought iron is 
least, and that of cast iron the greatest of the commonly 
used metals. Hard steel has great retentivity. 

161 



162 ELEMENTARY ELECTRICITY 

Question 5. How can you explain the facts of differ- 
ent permeability and retentivity? 

Answer. We know the following facts, and from 
these we have thought out an explanation which we 
think is correct. 

Facts :— 

(1) The softest iron is the most permeable. 

(2) Soft grey cast iron has greater permeability than 
hard white cast iron. 

(3) The harder the steel the less permeability. 

(4) When a bar of iron or steel is suddenly mag- 
netized there is a faint click and the bar lengthens slight- 
ly. If magnetized and demagnetized rapidly the clicks 
will merge into a hum or buzz. Keeping this up for a 
length of time heats the bar. 



Fig. 74. The result of Breaking a Magnet is Several Short Magnets. 

(5) Pounding or jarring a permanent magnet 
weakens it. So does heating it red hot. 

(6) If a magnet is broken, each -piece is a perfect 
magnet with two poles. No matter into how many 
pieces you break a magnet, each is still a magnet. See 
Fig. 74- 

(7) If a thick piece of steel is magnetized and then 
laid in nitric acid for some time, when the outer surface 
has been eaten off, a test for magnetism will show that 
the bar has lost almost all its polarity. Magnetism is 
evidently only skin deep. 

(8) Four bars 6xix)4 inches magnetized and 



MAGNETISM — CONTINUED 163 

bound together make a much stronger magnet than one 
bar 6 x I x i magnetized from as powerful a source. 

Theory : — 

From the above facts we have concluded that a piece 
of iron or steel is not solid but comprised of innumera- 
ble small particles which are each a perfect magnet. 

In ordinary iron or steel these are all jumbled up as 
in Fig. 75, and do not show any magnetism. 



I 



Fig. 75. A Bar of Iron Unmagnetized. 

There being about as many north and south poles 
pointing the same way, they neutralize each other. 

Now suppose this bar be stroked by a magnet as in 
Fig. 65. The influence of the magnet will be strongest 
at the surface and weaker as it penetrates. In fact, after 
^-inch under surface the action is practically nothing. 




Pig. 76. A Bar of Iron while magnetized. 

What this magnet does is to pull all the little particles 
around into line with the north poles pointing one way 
and the south poles the other. This movement of the 
particles causes a slight noise if they move all at once, 
but the gradual movement due to the stroking does not 
produce a sound that we can hear. If the rod is iron it 
lengthens a tiny bit; if it is a nickel rod it shortens a 
little (about 1-700000 of its own length). 



164 



ELEMENTARY ELECTRICITY 



The rapid magnetizing and demagnetizing pulls them 
around so fast that the internal friction heats the bar. 

Fig. 76 shows the bar with the particles all in line, and 
as each particle is a miniature magnet, Fig. yy shows 
why the whole bar becomes a magnet. 

If now the magnet be pounded the vibrations shake 
up the particles and they get jumbled up and the bar 
ceases to show its magnetism. 

The harder the iron or steel the more difficult it be- 
comes to pull the particles into line and make a magnet. 

In the same way once magnetized the better they stay 
in position. 



S 



o i< :rai<i«j*xa«i«irra:ra:inrBi< :n 



Fig. 77. Internal Structure of a magnetized Bar. 



Very soft wrought iron may be magnetized easily, but 
as soon as the magnetizing force is removed the parti- 
cles slip back to the jumbled condition. 

A glass tube filled with cast iron filings may be mag- 
netized by a coil of wire. On examination the filings 
will be seen all arranged, end for end. 

Handled carefully it will act as a magnet but when 
shaken so as to jumble up the filings it loses its polarity. 

You notice we do not say it loses its magnetism, for 
it does not. It ceases to have magnetic poles at each 
end, i. e., it has lost its polarity. 

When we say demagnetize a bar, it would be more 
accurate to say depolarize. 

We use this word, however, for a different thing and 
say demagnetize, for every one understands what we 
mean. 



MAGNETISM— CONTINUED 165 

Question 6. Will a magnet floated on water by corks 
be drawn to north or south? 

Answer. No. The force that the earth exerts on a 
magnet. will Ortly turn it into the north and south line. 

The poles of the earth are so far away and the magnet 
so small that the distance between the two poles of the 
magnet is practically nothing as compared with the dis- 
tance from magnet to either pole. 

The sOuth magnetic pole of the earth attracts one end 
and repels the other end of the magnet. The forces are 
equal because distances are equal. The same thing hap- 
pens at other end of the magnet. The result is no mo- 
tion. 

Question 7. Can the earth's tendency to turn a com* 
pass needle be neutralized? 

Answer. Yes» Take a bar magnet and hold it high 
above and parallel with the compass needle. Let its 
north pole point in the same direction as the north pole 
of the compass needle. 

Slowly lower the magnet until the compass needle 
starts to waver. If the magnet is fastened in this posi- 
tion the compass needle will stay in any position it is 
placed. 

Question 8. Is this of any practical use? 

Answer. Yes. If surrounding iron objects or mag- 
netized things are interfering with the earth's effect on 
the compass, then by means of extra magnets these dis- 
turbing influences may be neutralized and the earth's 
magnetism alone left free to turn the compass. 

This must be done with compasses on iron or steel 
ships. 

Question 9. What is meant by magnetic force? 

Answer. The force exerted by one magnet on an- 



166 



ELEMENTARY ELECTRICITY 



other to attract it or to repel it, or to attract iron filings 
or pieces of iron is termed magnetic force. 

Question 10. Does this force act all over the magnet? 

Answer. There is almost none in the center, and most 
at the ends. Plunging the end of a magnet into a box 
of iron filings shows this. 

Question ii. What are the relative magnetic forces 
at different points from the center to the end of a mag- 
net? 




Fig. 78. Strength of Magnetism as roughly Tested by Lifting Power. 



Answer. Fig. 78 shows approximately the magnetic 
force at different points. It shows that the pole of a 
magnet is not at the tip end, but a little way back. 

An accurate test of a six-inch bar magnet has shown 
these results: 



Location of Point. 


Magnetic Strength. 


Center 


Kone 




%" away 


9 units 




1 


20 




1% 


83 


Other end gave 


2 


48 


same results 


W 


65 




2X Pole 


84 




3 Tip end 


80 





MAGNETISM — CONTINUED 



167 



Question 12. What are the magnetic lines of force? 

Answer. These words are used in several different 
ways. The word magnetic is usually dropped for the 
sake of shortness and "lines of force" spoken about; 
Engineers usually say "lines." 

(i) The magnetic force of a magnet seems to lie 
along certain lines and these lines are called "lines of 
force." Perhaps "direction of force" would be a better 
name. 

(2) When a magnetic pole exerts a certain force of 
repulsion on the same named pole of a magnet of equal 




Lines of Magnetic Action around a Bar Magnet. 



strength we say that there are 10,000 "lines of force" 
to the square inch in that magnet pole. If it exerted 
twice that force we would say there were 20,000 lines to 
the square inch. 

Perhaps "units of force" would be a better name for 
this. 



168 ELEMENTARY ELECTRICITY 

It has been proposed to drop the expression "line of 
force" when used as "units of force/' and say 10,000 
gausses. Whether this word will be adopted of not re- 
mains to be seen. 

Question 13. How may the direction of force be 
ascertained? 

Answer. Place the bar magnet on a table (Fig. 79) 
and lay over it a sheet of glass. 




Fig. 80. The Magnetic Field of a Bar Magnet as shown bj 
Iron Filings. 



Lift iron filings over the glass and tap very gently. 
The filings will arrange themselves and show the direc- 
tion of the magnetic force at all points. 

Many different combinations of poles should be tried. 
Figs. 80 and 81 show two magnetic spectrums. 



MAGNETIC INDUCTION 169 

Question 14. What is a magnetic field? 

Answer. The space around a magnet under its in- 
fluence is called a magnetic field. 

The spectrum made with iron filings shows the direc- 
tions of the magnetic forces in the magnetic field. 




Pig. 81. The Magnetic Field of a Magnet Pole. Magnet at Right 
Angles to Observer. 



Question 15. What is magnetic induction? 

Answer. When a piece of soft iron free from mag- 
netism is placed in a magnetic field it becomes a magnet 
by induction. 

Fig. 82 shows this. The part of the iron under the 
north pole of the magnet becomes a south pole. 

It may be difficult to find a piece of iron which is not 
slightly magnetized. 



170 ELEMENTARY ELECTRICITY 

Question 16. How can you demagnetize? 

Answer. Heat to a cherry red and cool very, slowly. 
The piece may be now hardened and tempered and no 
magnetism will appear. 

Question 17. Explain magnetic induction more fully 

Answer. (1) Hold a magnet horizontally and at- 
tach to its north pole a soft iron nail. To this nail at- 




y — ~xyw?Z — 7 



Iron Filmy* 

Fig. 82. The Induction of Magnetism in Iron without Contact. 



tach another until three or four are adhering to the mag- 
net. The end of the first nail which is in contact with 
the north pole of the magnet becomes a south pole by 
induction. This temporary polarity of the first nail acts 
on the other, and so on down the line of nails. The po- 
larity of the nails is shown by small letters on the left 
side of the nails in Fig. 83. 

Now slide the south pole of a similar magnet (the 
other one of the pair) over the north of the first magnet. 

This magnet will act inductively on the nails, and the 



MAGNETIC INDUCTION 171 

small letters on the right side of the nails show the polar- 
ities induced. 

The result of the two effects is to render the nails 
neutral, and they drop off. 

(2) Attach nail to one magnet as before and then as 
in Fig. 84 place the other magnet below. The effect of 
the upper magnet is the same as before. The lower mag- 
net acts inductively on the nails, but the south pole of 
the magnet acts on the lower end of the last nail, so 
that the polarities induced are the same as those induced 
by the upper magnet. The result is a stronger magnetic 



» ■« ==■ 



Fig. 83. Demagnetizing Inductive Fig. 84. Increased Magnetizing 
Effect of Unlike Poles. Effect of Unlike Poles. 



action than before. It will be seen by experiment that 
more nails can be supported. 

Only one set of small letters is used because both mag- 
nets induce the same polarities. 

(3) If, as in Fig. 85, the two magnets are held to- 
gether, we have the same effect as if a stronger magnet 
were" used. 

(4) Placing the second magnet below as in Fig. 86 
results in demagnetizing the nails. The left-hand let- 
ters show effect of upper magnet, the right-hand letters 



172 ELEMENTARY ELECTRICITY 

the effect of the lower magnet. Net effect, no mag- 
netism. 

A slightly different explanation of these four facts 
will be given in Lesson 13. This explanation will be no 
better, but merely tellino- the same thing in a different 
way. 

Question 18. Is there any material which will in- 
sulate from magnetism? 

Answer. No. Deflect a compass needle slightly by 
a bar magnet. Slip in between them thick sheets of 
cardboard, copper, wood, glass, rubber, in fact anything 
but iron or steel, or perhaps a thick slab of nickel, and 
the deflection of the needle is not affected. 



91 
* 



S TSfr 



l>* *» 



X 

1"' fi» 



Fig. 85. Increased Magnetizing Fig. 86. Demagnetizing Inductive 

Effect of Like Poles. Effect of Like Poles. 



This shows that magnetism passes through anything. 

Question 19. What would be the result if a piece of 
iron or steel were placed between needle and magnet? 

Answer. If a sheet of steel, iron (wrought or cast), 
is interposed the deflection will become much less. 

Question 20. How do you explain this? 

Answer. It seems that everything conducts mag- 
netism equally well, but perhaps equally poorly would 
be more accurate, except iron or steel. Therefore the 
magnet does not care or even know that something else 



MAGNETIC SCREENING 



173 



has been put in place of part of the air through which 
it must send its force or effect. But when the iron is 
put in place it is so much better a conductor that the 
magnetic effect prefers the easy path through the iron off 
to the side, instead of going on into the air beyond. 
Hence only a little effect passes on to the compass. In- 
deed, if the iron screen were thick enough in proportion 
to the strength of the magnetism present, none would 
pass through. 



A 



CO 



V 






*xi 



i Is* 



Jf n S— , 'fog tjneHc Body 



__ 



Zints 

JVom Earth's 
Jf.Fole 

Lines 

JTo Earths 

S.Pole' 

Fig. 87. Needle Deflected Through a Non-Magnetic Body, 




These effects are shown in Figs. 87 and 88. The lines 
along which the magnetic force acts are also shown. 

Question 21. Are there any practical uses of the 
screening action? 

Answer. Yes, several uses are made of it. 

( 1 ) Advantage is taken of this fact by putting an ex- 
tra hunting case of soft iron on a watch to screen it from 
the magnetism of electrical machinery. 

(2) Measuring instruments used on switchboards or 
near dynamos are enclosed in heavy cast-iron cases. 



174 



ELEMENTARY. ELECTRICITY 



(3) Galvanometers are sometimes surrounded by a 
cylinder of wrought iron, or a piece of heavy iron pipe. 
A small hole is cut in the side, so as to observe the de- 
flection of the needle. 

Question 22. To what practical use are permanent 
magnets put? 




Pig. 88. 



Needle Screened from Action 
Body. 



~*i> Earth t 
S.Pole 



of a Magnet by a Magnetic 



Answer. They are used in the receivers of tele- 
phones; in magneto bells for telephone calling and gen- 
eral signalling; in the magnetos used for ignition pur-. 
poses in gas, gasolene and oil engines; as magnets in 
measuring instruments. 

Question 23. How are magnets named? 

Answer. When an engineer or electrician speaks of 
a magnet he means an electro-magnet ; when he says per- 
manent magnet he means a piece of hard steel which 
has been magnetized. In referring to a piece of iron 
temporarily magnetized he calls it an armature or a core. 



LESSON 12. 

Electro-Magnetism. 

It is well known that all the effects due to a natural 
magnet or an artificial magnet, can be produced by a 
current of electricity. 

The effects produced by the electric current are so 
much more powerful than natural magnetism, and the 
cost is so much less that they are almost universally used. 




Fig. 89. Magnetic Field due to Electric Current. 



Question I. What is electro-magnetism? 

Answer. It is the magnetic effect produced by a flow 
** electric current. 

Question 2. Does a wire carrying current have a 
magnetic field? 

175 



176 ELEMENTARY ELECTRICITY 

Answer. Yes. Pass a straight wire carrying a large 
current through the center of a sheet of cardboard as 
shown in Fig. 89. If iron filings are sprinkled on the 
board they will arrange themselves in circles around the 
wire, thus showing the magnetic field. 

Question 3. Will the wire affect a magnetic needle? 

Answer. Yes. If a small compass be placed on the 
cardboard and pushed around the wire it will be seen 
that the needle is being moved by the magnetic action of 
the current in the wire. 







Fig. 90. Action of Electric Current on Magnetic Needle. 

It is also easily shown by holding a wire carrying 
current over a magnetic needle. The needle will be 
turned and stand at an angle to the wire. Fig. 90 shows 
this. 

Question 4. Does the wire exhibit north and south 
polarity like a bar magnet? 

Anszver. No, not like a bar magnet. It moves the 
magnetic neeclje in the following way: When the cur- 



§MOW SULfe 



111 



rent flows from the South to North Over the needle 
the N-end turns West. 

This is called the SNOW rule, so called from the first 
letters of the important words of the rule. 

To test this rule arrange things as in Fig. 91. Turn 
the cell so that a line through the copper and zinc 
plates will be north and south, and have the copper plate 
to the south. Bend your wire circuit into a loop stand- 
ing vertically in the N. and S. line. Stand behind the 
zinc plate facing south; put a small compass in this 
loop with the N. mark on the box pointing to the north. 




Fig. 91. The S-N-O-W Rule. 



If the circuit is open, i. e. the dry ends of the plates 
not touching, and the connecting wires cut and the ends 
held apart, then the N-end of the needle will as usual 
point to the north. 

Close the circuit by touching the ends of the wire, 
and the N-end of the compass will move to the west 
(to your right hand). Since we say the current flows 
from carbon to zinc plates, this proves our rule cor- 
rect 



178 ELEMENTARY ELECTRICITY 

Question. 5. Give a further proof that although the 
current acts magnetically it does not act like a bar mag- 
net. 

Answer. If the wire were a simple bar magnet put- 
ting the current under the needle would not change the 
direction of the deflection, but it does change the direc- 
tion of the deflection. Further if the wire were like a 
bar magnet when the current was reversed it would 
change in polarity and attract the needle holding it in 
the north and south line, but it does not do this. When 
the current is reversed it deflects the needle the other 
way. 

Question 6. Why does the current act in this way 
on a magnetic needle? 

Answer. We do not know. The current acts as if 
it had a paddle wheel of magnetism rotating on the 
wire as an axle and in the opposite direction to the 
hands of a watch when the current is flowing towards 
you. 

Standing in the position shown in Fig. 91, when the 
current is over the needle flowing towards us (north), 
the part of the whirls (paddle wheels) of magnetism 
on the under side of the wire are turning towards the 
west and knock the N-end of the needle in that direc- 
tion, and since the S-end always does the opposite thing 
it goes east. 

But moving the wire under the needle the parts of 
the whirls on top of the wire are moving east and push 
the N-end of the needle in that direction. 

Fig. 92 shows the magnetic whirls around the wire, 
and the effect of the SNOW rule. For since the cur- 
rent runs in the opposite direction from the rule, we 
should expect to find the N-end of the needle move in 



RIGHT HAND RULE 



179 



the opposite direction. This is exactly what it does. 
The N-end of the needle moves East. 

This may be made into a rule and called the' NOSE 
rule. 

When a current flows from North Over a needle to 
the South the N-end is deflected East. 

Question 7. For what is the extra hand in Fig. 92? 

Answer. The extra right hand shown in the illus- 
tration is another way of remembering the SNOW rule, 
and is especially adapted to discover in which direction 
the current flows. 



EaUcvj 




Dotted Circle* and Arrow* 
thereon indicate dire*tio% 
*."•"** ••iXi-7 of Current > Field of Fore 



Fig. 92. Action of Current on a Magnet. 



'Arrange the wire above the needle in a N and S line 
placing the palm of right hand over the wire with the 
thumb stretched out at right angles to the hand and 
pointing tozvards the n-end of the needle. The fingers 
point in the direction the current Hows. 

Question 8. What is an electro-magnet? 

Answer. An electro-magnet consists of a coil of wire, 



180 



elementary electricity 



a piece of soft iron to fill the hollow of the coil and a 
current to pass through the wire. 

Fig. 93 shows this, as well as the lines of magnetic 
action. 

Question g. What are the technical names of the 
parts of an electro-magnet? 

Answer (i) The iron is called the core. 

(2) The coil of wire is called a helix. 

(3) The helix carrying current (without a 
core) is called a solenoid. 




Fig. 93. An Electro-Magnet with its Magnetic Field. 



(4) A core placed in a solenoid makes it an 
electro-magnet. 

Remember plugs of brass, fiber or other materials 
placed in a solenoid are not cores in the technical sense 
of the word. 

(5) The helix or coil is usually not wound on 
the core but on a spool of brass or bronze. The ends 
of this spool are called the flanges and show in Fig. 93. 

(6) On horse shoe magnets the iron connecting 
the two cores is called the yoke. 



MOLARITY OF MAGNET 



lSl 



Question 10. Why is the left end an N-pole and the 
right end a S-pole. (In Fig. 93). 

Answer. The polarity of a magnet depends on the 
direction of the current flow through the helix. 

In Figs. 94 and 95 the current enters at the right hand 
end of the helix, but being wound, one right handed and 
the other left handed the polarities developed are as 
shown. 




Fig. 94. Electro-Magnet with Right-handed Helix. 

Question II. What rule will give you the polarity of 
a magnet? 

Answer. Hold the magnet so that the current flows 
through the helix away from you. If the current flows 
round the core in the direction that the hands of a watch 
or clock move, the pole you are looking at is an S- 




Pig. 95. Electro-Magnet with Left-handed Helix. 



pole. If the current went counter-clock wise around the 
core, the pole is an N-pole. 

Question 12. Is there any other rule? 

Answer. Yes. It is a rule especially applicable to 
winding horse shoe magnets so that both legs of the 



182 



ELEMENTARY ELECTRICITY 



magnet will not be accidentally made of the same po- 
larity. 

Write on the pole piece the letter S or N as the case 
may be and put arrow heads on the ends of the letters 
as in Fig. 96. These arrow heads show the direction 
the current must flow around the core to give the po- 
larity indicated by the letter. 




Fig. 96. Rule for Proper Winding of Horse-shoe Magnets. 



Question 13. What is the strength of a magnet? 

Answer. The term "the strength of a magnet" 
is used very carelessly. Some people mean its lifting 
power, while engineers and electricians mean the actual 
quantity of magnetism flowing from a pole piece.* The 
expression "strength of a magnet" should only be used 
as engineers or electricians use it. 

Question 14. What is the lifting power of a mag- 
net? 

Answer. It is the number of pounds the magnet will 
hold up, when things are arranged as in Figs. 97 and 

98; 

*In electro-magnets when we say pole we generally mean the 
end of the core. Usually we say pole piece, meaning the surface 
at the end of the core, or the piece of iron which is screwed, 
bolted or even cast on the core to make the actual pole largei 
than the core. 



LIFTING POWER 



183 



There are some very curious things about lifting 
power. It depends upon the shape of the magnet, and 
the actual quantity of magnetism. A horse shoe mag- 
net will lift three or four times as much as a bar magnet 
of equal strength. A long bar magnet will lift more 
than a short one; the actual quantity of magnetism be- 
ing the same. 




Fig. 97. Horse-shoe Magnet 

Arranged for Test of Lifting 

Power. 




Fig. 98. Testing Lifting 
Power of a Solenoid. 



A magnet with round or pointed pole pieces will lift 
more than the same magnet with flat or broadened pole 
pieces. The same magnet may have its lifting power 
changed by unscrewing its pole pieces and screwing on 
a flatter set. 

It is an excellent joke to wind a horse shoe magnet 
with one pole piece flat and the other rounded. If one 
will hold 5 pounds the rounded one will hold 6 pounds. 



184 ELEMENTARY ELECTRICITY 

A magnet having its armature loaded almost to the 
pulling off point, may have its load increased slightly 
the next day. This gradual increase of the load may be 
attempted every day until finally the armature will be 
torn off. 

It will be found that the last day the magnet was 
holding up a load that it could not have held had it 
been applied all at once. This may be easily proved by 
trying a load a little less than that which tore the arma- 
ture away. The magnet will not be able to hold it. 

Doubling the strength of the same magnet more than 
doubles its lifting power, but a magnet will not pull 
twice as strongly if you move up twice as close to it. 

A test made with the armature in contact with the 
poles of the magnet with increases in the magnets' 
strength gives practically the following results for lift- 
ing power. 

Do not forget that doubling the current in an electro 
magnets coil does not double the strength. This will 
be fully explained later. 

Test i. 



Relative strength 


of Magnet. 


Relative 


lifting power. 


i 






i 


2 






4 


3 

4 






9 

16 


5 
6 






25 

36 


7 
8 

9 






49 
64 
81 


10 


- 




100 



LIFTING POWER 185 

Test 2. 

A test made with a horse shoe magnet to determine its 
lifting power at different distances from its poles shows 
the following results: 



Distance away 


from 


poles. 


Lifting power, 


o 






82.0 


I 






35-o 


2 






25.0 


3 






20.0 


4 






15.1 


5 






12.1 


6 






ii.3 


7 






9-3 


8 






74 


9 






6.5 


IO 






5-5 



LAW OF DIRECT AND INVERSE SQUARES, 

The results of Test I are not the actual figures in the 
test. They have been increased or diminished a little 
in order to illustrate what is called the Law of Direct 
Squares. 

By looking at the table of results you will notice that 
the lifting power is always the strength of the magnet 
multiplied by itself. For instance when the magnet was 
5 times as strong as before it lifted 5x5 or 25 times as 
much. 

When the magnet strength was increased from 1 to 2 
the lifting power was increased from 1 to 4. Doubling 
the strength quadrupled (2x2) the lifting power, 



186 ELEMENTARY ELECTRICITY 

With a magnet 5 units strength, doubling its strength, 
quadrupled its lifting power; increased it from 25 to 
100. 

We call a number multiplied by itself a square. The 
number which is multiplied we call the square root. 

To find the lifting power of a magnet square its 
strength. 

Having found this rule we could go farther than the 
results of the test and predict that if the magnet had 
been increased in strength to 12 times the original 
value, the lifting power would be 12x12 or 144. 

The greater the strength of the magnet the greater 
the lifting power. This is called a direct relation. 

The complete rule as stated in text books is : 

The lifting power of a magnet varies directly as the 
square of its strength. 

Varies means changes. 

Directly means they both increase. 

Square means multiply strength by itself. 

The results in Test 2 do not seem to follow any rule. 
Even if the figures were changed a little as in Test 1 
they could not be made so as to get a rule from them. 

It is stated in nearly all text books that the lifting 
power of a magnet decreases according to the square 
oi the distance from the magnet. They state the rule 
thus: 

The attraction of a magnet varies inversely as the 
square of the distance. 

This rule is true with very tiny magnets at small dis- 
tances and in a space absolutely neutral, free from any 
magnetism, even the earth's effect. 

Under the circumstances of ordinary life this rule is 
worthless. 



LAW OF INVERSE SQUARE 187 

What the test shows is this: — As you recede from a 
magnet the lifting power decreases at first rapidly and 
;hen more slowly. 

The word inversely in the rule means the greater 
the distance the less the lifting power. 

The inverse square of a number is found by squaring 
the number and placing it in the denominator of a frac- 
tion whose numerator is I. 

Example : 
Numbers 12 3 4 5 6 etc. 

Direct Squares 1 4 9 16 25 36 et 

Inverse Squares 1 \ i rg- Yt -gr etc. 

The peculiar thing about I is that ixi is still I and 
that the fraction i-i is still I. 

The importance of these laws of direct and inverse 
squares is greatly magnified. At the best magnets do 
not follow the first rule closely, and they do not follow 
second rule at all. 



LESSON 13. 

Electro-Magnetism — Continued. 

Question I. What is the "strength" of a magnet, 
speaking in a correct manner? 

Answer. The actual quantity of magnetism flowing 
through one square inch of the surface of the pole piece. 

Question 2. How is this quantity of magnetism meas- 
ured? 

Answer. Scientists have selected for a unit of mag- 
netism a quantity they call a "line." 

Their tests for the presence of "lines" and the deter- 
mination of how many "lines" there are* in a magnet, 
need not worry us, as the engineer's job is usually to 
produce "lines." 

Take a stick of wood one inch square, wrap around it 
a piece of wire making one complete turn and no more. 
Pass a current of one ampere through this turn of wire 
and you will produce a flux of a little over 3 lines. 

Question 3. What does Flux mean? 

Anszi'er. The term flux is used to speak about the 
total quantity of magnetism. For example, a designer 
will say that the flux from a magnet is 3^2 million lines. 

Question 4. What is meant by Density? 

Answer. By density we mean the flux per square 
inch. A designer may say that both these magnets have 
a flux of 2 million lines ; but this one having a density 
of 10 thousand lines has a smaller core than the other, 
the density of which is 5 thousand lines. 

188 



FLUX OF MAGNET 189 

To get the flux of a magnet multiply the density by 
the area of the pole piece in square inches. 

Knowing the flux we find the density of any part by 
dividing the flux by the area of that part in square 
inches. 

Question 5. What is meant by Intensity of magneti- 
zation ? 

Answer, It is an out of date term among engineers. 
A man using the expression probably means density. 

Question 6. What is meant by Magnetizing Force? 

Answer. Magnetizing force or Magneto-motive force, 
means the force that is causing magnetism to flow out 
of the pole piece. In other words magnetizing force is 
the cause of flux. 

Question 7. How is magnetizing force measured? 

Answer. Since one turn of wire carrying one ampere 
current causes a definite flux, we use this as the unit to 
measure magnetizing force. It is called the Ampere 
turn. 

Experiment shows that half a turn of wire and two 
amperes cause the same flux as three turns and one 
third of an ampere. In fact, an ampere turn is any com- 
bination of turns and currents arranged so that the num- 
ber of turns multiplied by the current in amperes gives 
a product of one. 

Question 8. Upon what does the flux from a magnet 
depend ? 

Answer. (1) Material of core. 

(2) Length of core. 

(3) Number of turns of wire in coil. 

(4) Current in coil. 

(5) Shape of magnet, 



190 ELEMENTARY ELECTRICITY 

Question 9. Why does the material of the core affect 
the magnet? 

Answer. To answer this clearly let us go back a little. 
A solenoid such as shown in Fig. 99 has a certain flux 
whose density is calculated by the following rule. 

The density is equal to the number of Ampere-turns 
multiplied by 0.313 and divided by the length of the 
solenoid in inches. 



y ? 




Fig. 99. Flux in a Solenoid. 

If the solenoid is very short the density at the ends 
may be much less than in the middle. 

Let us assume that we have a solenoid with a central 
space one square inch in area and of such a number of 
turns and carrying such a current that it has 3133 A. T. 
(ampere- turns) to each inch of its length. 

Since one A. T. gives a flux of about 3 lines per 
square inch (see A 2), these 3133 A. T. will give a flux 
of 10,000 lines. 

Let us take a solenoid like Fig. 99 and slip a wrought 
iron core into it making a magnet as in Fig. 100. Have 
the area of the bars' end one square inch (a bar i l /% 
inches in diameter has a cross section of practically 1 
square inch). 

Let the magnet have such a helix and carry such a 
current that there are 2.2 A. T. per inch of its length. 



EFFECT OF CORE 191 

The flux of this magnet will be 10,000 lines. 

This experiment can be completed by making a magnet 
of 1 }i inch diameter cast iron rod with 18.5 A. T. per 
inch, and the flux will again be 10,000 lines. 

It is quite evident then that the core has great in- 
fluence on the strength of the magnet. 




Fig. 100. Flux in a Magnet: i. e., a Solenoid with Iron Core. 



The explanation is that wrought iron has far greater 
permeability than air and so 2.2 A. T. per inch can in- 
duce in the wrought iron as much flux as 3133 A. T. per 
inch could induce in air. 

The permeability of cast iron being less than wrought 
iron it takes 18.5 A. T. to do the work that 2.2 A. T. 
did before. 

Question 10. How does the length of the core affect 
the flux? 

Answer. In this way 500 A. T. wrapped on an 1% 
inch round iron* bar one inch long would give a flux of 

*\Vhen we speak of iron we mean in general wrought iron or 
Bessemer steel. When we mean cast iron we generally say so. 

Bessemer steel is cast iron with the extra carbon burnt out. It 
is nearer to wrought iron than any other metal. 



192 ELEMENTARY ELECTRICITY 

115,000 lines; while if the bar was 10 inches long there 
would only be a flux of 90,000 lines. 

Question II. Why does not the same number of A. T. 
give the same flux ? 

Answer. Because the iron although a good conductor 
of magnetism, offers some opposition to the flux. So if 
500 A. T. can induce in a core one inch long, a flux of 
11,500 lines, in a longer core it is to be expected that 
the flux will be less. 

The magnetizing force of 500 A. T. can only do a cer- 
tain amount of work and the longer the path the flux 
must be forced through, the less flux there will be. 

To get the same flux through different lengths of cir- 
cuit the A. T. per inch of circuit must be the same. 

Question 12. Well then, if 500 A. T. give a flux of 
115,000 lines through 1 inch of iron, why did they not 
give 11,500 or 1/10 of 115,000 through the 10 inch piece? 
Instead of that you say 500 A. T. on a 10 inch piece give 
a flux of 90,000 lines. 

Answer. The reason is that while iron is more per- 
meable than air the exact degree of permeability depends 
on the density. 

500 A. T. on a 1 inch piece is 500 A. T. per inch. 

500 A. T. on a 10 inch piece is 50 A. T. per inch. 

Five hundred A. T. per inch can only create a flux of 
11,500 per square inch because the density is so high that 
the iron offers a great deal of opposition to the flux, 
while 50 A. T. per inch, not being strong enough to 
create a great flux, finds the iron offering less opposition 
and is able to create a flux per square inch of 90,000, or 
nearly eight times what you would expect. ' 

Question 13. Why does iron offer different opposi- 
tion at different degrees of magnetization? 



EFFECT OF DENSITY 193 

Answer. Why iron should need greater and greater 
increases of magnetic force to produce' the same increases 
of density (flux per square inch) we do not know. It 
does not even act in any regular manner. 

It is easy to show that this does occur, and perhaps we 
can understand the peculiar action a little from the fol- 
lowing experiment: 

Procure a thick short elastic band and a bunch of 
butcher's wooden skewers. 

Place a dozen skewers inside the band points down. 
No effort is required ; they practically fall in. Place an- 
other dozen in. Perhaps a gentle pressure is necessary 
as you begin to feel the pressure of the band. 

The next dozen must be pushed in. The following 
dozen you have to push in one at a time. At last you will 
have to drive the skewers in, one at a time, with a block 
of wood or a light hammer. 

Evidently the ease with which the skewers can be 
placed inside the band depends on the number of skewers 
per square inch you are trying to get in. As the density 
increases the difficulty of insertion increases. 

The iron acts in the same manner. The more flux in 
a core the greater a magnetizing force is necessary to 
place additional lines in it. 

Question 14. Do all magnetic materials act in this 
way? 

Answer. Every magnetic material acts in this way in 
its own peculiar fashion. 

Question 15. How do the non-magnetic materials 
such as brass, air, fibre, etc., act? 

Answer. Non-magnetic materials act in an ordinary 
manner. Twice the magnetizing force produces twice 
the density. 



194 ELEMENTARY ELECTRICITY 

Question 16. What statement can be made about per- 
meability of materials? 

Answer, (i) The materials which are in common 
use are here arranged according to their permeability at 
a density of 10,000 lines. The first has the greatest 
permeability. 

(a) Annealed wrought iron. 



. practically equal. 

(b) Soft steel castings. ) r 

(c) Cast iron with a little aluminum. 

(d) Ordinary grey cast iron. 

(e) Air, fibre, brass, zinc, copper. (All practically 
equal and very low.) 

(2) At first the magnetic materials (mentioned in a, 
b, c, d) give more than twice the density for twice the 
magnetizing force. Then they change and act regularly 
for a short time. After this they give rapidly decreas- 
ing increase of density for equal increases in the mag- 
netizing force, till at last a doubling of the A. T. per 
inch hardly increases the density. We then say the ma- 
terial is saturated. 

(3) They keep the same order of degree of perme- 
ability but their actual permeabilities change in different 
manners. 

At 10,000 density a steel casting is 5 times as permeable 
as grey cast iron, while at 60,000 density steel castings 
have 18 times the permeability of iron castings. 

(4) The permeabilities of all non-magnetic materials 
may be taken the same as air without much error, and 
their permeabilities at all densities are the same. 

Question 17. How can definite information as to the 
permeability of a metal be obtained? 

Answer. A specimen is cut and tested in a laboratory. 
For all ordinary use the average results as published in 



TABLE 



195 



AMPERE TURNS REQUIRED PER INCH LENGTH 

TO INDUCE THE FOLLOWING DENSITIES : 

A T per Inch. 



Density. 


Soft Iron. 


Soft Steel 
Castings. 


Cast Iron. 


Air. 


5000 


1.7 


2. 


13. 


1566 


10000 


2.2 


3.7 


18.5 


3133 


15000 


2.7 


4.3 


24.1 


4700 


20000 


3.5 


5. 


30.5 


6266 


25000 


4.5 


5.8 


39. 


7833 


30000 


5.5 


6.6 


50. 


9400 


35000 


6.5 


7.6 


65. 


10966 


40000 


7.5 


8.8 


88. 


12532 


45000 


8.5 


10.1 


116. 


14100 


50000 


9.6 


11.8 


160. 


15665 


55000 


11.1 


13.9 


222. 


17233 


60000 


13. 


16.4 


295. 


18800 


65000 


15.7 


19.3 


400. 


* 


70000 


19.6 


22.7 


570. 




75000 


24.7 


27. 


* 




80000 


31.2 


34. 






85000 


39.7 


44. 






90000 


50.7 


57. 






95000 


67. 


75. 






100000 


91. 


100. 






105000 


137. 


159. 






110000 


290. 


325. 






115000 


500. 


550. 






120000 


* 


* 






125000 











*The figures are not given when the number of A T becomes 
excessively large or beyond the usual limits of density. 

Air gaps are generally worked at a density below 50,000. 

A density of over 105,000 is rarely used in iron or soft steel. 

To find the AT corresponding to a density not given: — 

How many A T per inch are required for soft steel at den- 
sity of 37,000. 



40000 

35000 

5000 

1000 

2000 

35000 



takes 
takes 



8 8 1 

y] 6 J- Subtract 

TIT Divide by 5 

0.24 Multiply by 2 

0.48 Add in 
7.6 



37000 takes 8.08 A T per inch. 

This scheme is called interpolation and can be applied to any table. 



1 r - - 



EL Wiener's book on dynamo designing. 
Question 18- What is Refnctance? 

-.'-.-. :t_ iritre: : ;. :.t:i :: ~i:r~i_ :: :ht :i:fi^t 
: : n _"••: 

J ' : -/-'/; : :. '.'.'zi'. :r ?.i".-.::r-v;77 : 

.-: .: 3 .■:'..::-..--."■ : :hr rtl^:-:ir_:-e :: i : t:t :: 

:;.::-.... :r.t ir.zJ: !: : .^ in: ir.-: i::.i:t sir. :- _-_.: -:- 



I ■::: _: " irt :.ir :— : -"ir:- :r:^-ir- : 
^ciw^r. Because it is necessary to eapw s s the rdnc- 
-;-.;r: :: rr.irr.t: : : ':.:.:: .: :• ri.i^; zt:-air :: 

. - .-;..:.: 

:trrr_: ~i:tr i'i 7. :; :h;f t ;.:.: '~2 r :: .::: :hr 
expression "remrtance per cubic inch"' unless we use 
rt- .: :::.'■ :r<\ 

Question 21. Way do we not hear die word rehv> 
tfv.r. .;f-i ~:re ::r:.::r.:.; ■ ' 

l 
rondnrtivity rer cnbk inch instead of the opposition per 

T t ..:::.:; :: 1 ~.-\- ::' ~2:e~il me ir_:h im; 
=z : . ' ■. r : - :- * - : r. : - : ■ ; f - : : : r. . = : :i_ . t : : -5 : t :~ 1 1 
:..:; I: .i :hi: :.i"tr :r: . r..:"- " t .:•:. 
Question 22. What is Permeance? 
Answer. The name permeance is given to the total 
conductivity of a piece of material for fltrx. 

T : • . :; "":::.: - "rim ':; «.iy:r_z mi: rem:- 

Answer. When two things are opposite in sense, as 
relnetEvity and nermeabilitT. one being die Ji fpnMiin^ 



feESiDUAL MAGNETISM 197 

the other the conductivity, of the same piece of metal, we 
call them reciprocals of each other. 

If the permeability of iron is 200, its reluctivity is 
1 200, for the greater the permeability the less the re- 
luctarice: 

Question 24. What is Retentivity? 

Answer. A piece of iron becomes a magnet temporar- 
ily under the influence of ampere turns but loses nearly 
all its magnetism when the current is cut off from the 
helix. 

What remains is called Residual magnetism. The re- 
sidual magnetism per cubic inch is called the retentivity 
of the iron. 

Iron or soft steel has very little retaining power ; hard 
steel has great retentivity. 

Question 25. Is residual magnetism a good or bad 
thing ? 

Answer. It depends. In dynamo armatures we would 
rather not have it; in the yokes of the magnets we are 
very glad of it. 

In telegraph relays we try to reduce it as much as 
possible. 

A peculiar thing about residual magnetism is that a 
piece of soft iron which has been under the influence of a 
magnet only a thousandth of an inch away will show less 
residual magnetism than if it had been in actual con- 
tact. 

Telegraphers take advantage of this and paste tissue 
paper on the armatures of relays and sounders so that 
they may come very close to the magnetic cores and yet 
never come accidentally in actual contact. 
- Question 26. The word saturated was used in A 16. 
What does supersaturated mean? 



198 ELEMENTARY ELECTRICITY 

Answer. It is possible to magnetize a piece of steel so 
strongly that when tested instantly after magnetizing, it 
shows a strength in excess of what it will show four or 
five hours after. 

This second strength is its permanent strength. It can 
be magnetized permanently no stronger than this second 
strength, so this is called saturation. 

Question 2J. What is a magnetic circuit? 

Answer. From experiments which were first made 
with permanent magnets it seemed as if the flux of a 
magnet simply came out of each end. Later when the 
result of breaking a magnet was discovered, it was rec- 
ognized that the flux must also pass through the middle 
of the bar. 

What became of the flux after it left the poles was for 
a long time unknown. Experiments like Fig. 80 made 
electricians suspect that the flux which left one pole went 
around through the air and entered the other pole. 

In fact people soon began to believe that magnetism 
flows around a circuit just as electricity- does. 

In the case of the electric circuit it is all copper wire, 
while the magnetic circuit is usually composed of iron 
and air. 

Question 28. Can you give other reasons for believ- 
ing that flux flows around a circuit ? 

Answer. Yes. Make an electro-magnet of a ring of 
iron and a coil wound on as in Fig. 101 A. It will 
show no polarity at all, and be only slightly magnetic. 
Saw out a piece of the metal and the ring will develop 
polarity at X. and S. as shown in B. 

This shows that the magnetism flowed around the 
ring and also across the air gap when one was made. 



Molarity 



199 



Question 29. Why was there no polarity in Fig. 
101 A? 

Answer. Because the iron being a good conductor the 
flux passes through it and practically none is in the air 
where the testing needle was put. 

But if a hole should be bored in the ring and the 
Compass dropped into it. then the polarity would show, 
because flux would pass through the compass. 




Fig. 101. Magnetic Polarity of an Iron Ring. 



Question 30. Why did Fig. 101 B show polarity? 

Answer. Because the air gap being a poor conductor, 
the flux spread out as indicated and affected a compass 
needle brought near it. 

Question 31. Why should Fig. 101 C develop poles 
at opposite sides of the ring and why should it show so 
much magnetism in the air around it. 

Answer.. The left hand part of the winding produces 
a X-pole at top of ring. The right hand portion does 
the same thing. These North polarities oppose each other 
and force the flux out of the iron on both sides. 

Part of the flux passes directly across inside of ring to 
the S-pole and the rest curves around the outside of the 
ring through the air. 



§00 ELEMENTARY ELECTRICITY 

Question 32. What are closed and open magnetic cir- 
cuits. 

Answer. A circuit composed entirely of iron or mag- 
netic metals is called a closed circuit, while a circuit with 
an air gap in it is called an open circuit. Even if the air 
gap is filled with brass, fibre* etc., the circuit is still an 
open one. 

Figs. 101 B and C show open circuits. In C the iron 
is a continuous ring but there are two magnetic circuits 
each composed of a half ring of iron and an air space 
across which the flux passes. 

The results shown in Figs. 83, 84, 85 and 86 can be 
explained by magnetic flux and magnetic circuits, remem- 
bering that the permeability of even hard steel is many 
times higher than air, which means that its reluctivity is 
many times less. 

Suppose that in Fig. 83 only the first magnet with the 
nails hanging to its N-pole is present. The flux from the 
N-pole passes around the magnet to its S-pole. The su- 
perior permeability of the iron nails makes most of the 
flux pass to the S-pole through them. This makes them 
temporary magnets and they stick together. 

Now suppose the top magnet to be slid over the under 
one. The permeance of the magnetic circuit composed of 
the two magnets lying side by side is much greater than 
that of the circuit composed of one magnet, the nails and 
all the air from the end of the last to the S-pole of the 
first magnet. 

Naturally the greater part of the flux goes through the 
new path and the part of the flux through the nails is too 
small to strongly magnetize them. Their magnetism be- 
ing reduced they cease to adhere and drop off. 

The second magnet acts as a shunt circuit for the flux 



MAGNETIC CIRCUITS 201 

and being of great permeance robs the nails of the flux 
that magnetized them. 

In Fig. 84, when only the first magnet was there, we 
had rather a poor magnetic circuit — a piece of steel, the 
nails and a long air gap from last nail to S-pole of 
magnet. 

When the second magnet is put in position, the perme- 
ance of the circuit is improved because the air gap has 
been shortened. (Measure it and see.) Furthermore, in 
a circuit of greater permeance we have double the mag- 
netizing force. 

In Fig. 85, the adding of the second magnet increases 
the permeance and the magnetizing force. In Fig. 84 we 
reduced the air gap, while in Fig. 85 the air gap is left 
the same; hence the increase in inductive effect in Fig. 
84 is greater than in Fig. 85. 

In Fig. 86 the addition of the second magnet gives a 
result something like that shown in Fig. 10 1 C. The flux 
from the two N-poles which passes between the magnets 
is very weak and does not magnetize the nails sufficiently. 
The greater part of the flux goes back outside of the 
magnets. 






LESSON 14. 
Law of Magnetic Circuits. 

It is natural to suppose that there is some direct con- 
nection between the value of the magnetizing force and 
the flux induced. Also between the reluctance of the 
circuit and the flux induced. 

These three things: the magnetizing force, reluctance, 
and flux are connected in the following way : 

_ Magnetizing force. 
Reluctance. 

This formula while not of much practical use is of the 
highest importance theoretically. By that I mean : It is 
only by learning this formula by heart and understand- 
ing what it means that we can get a clear idea of how the 
flux in a circuit changes with the changes of magnetizing 
force, and how the changes of reluctance affect the flux. 

Let us once more be sure that we know what the terms 
used mean. 

Flux is the total amount of magnetism in the core, ex 
pressed as so many lines. 

Magnetizing Force or Magneto-motive Force is the to- 
tal pressure trying to send flux through the circuit. It 
is expressed by ampere turns multiplied by 1.25. 

Reluctance is the total opposition offered by the circuit 
to the passage of flux. 

The greater the magnetizing force the more flux; the 
greater the reluctance the less flux. 

202 



MAGNETIC LAW 203 

Let us see how this formula could be applied to a prob- 
lem in a designer's office. 

He would generally know what flux he wanted and he 
would know what kind of a circuit he intended to use, so 
he would want to find out how many ampere turns must 
be wound on the spools. 

He must find the reluctance of the circuit. Then know- 
ing two things he can twist the formula into this shape : 

Magnetizing force = Flux X Reluctance.* 
Flux X Reluctance 



Ampere turns = 



1.25 



The reluctance of a circuit is equal to the reluctivity of 
the material multiplied by the length and divided by the 
cross section of the circuit. 

This must be so because the reluctivity is the opposi- 
tion per cubic inch. The longer the circuit the greater 
the reluctance and the greater the area of the cross sec- 
tion the less the reluctance. 

In the first case the flux has further to travel and in 
the second case has more room to travel in. 

Calling L length of circuit in inches and a its area, the 

formula becomes: 

A Flux X reluctivity X L 

Ampere turns = 

1.25 X a 

This designer will not find any tables of reluctivity in 
books, but he can find tables or curvesf of permeability. 
Reluctivity is the reciprocal of permeability. 

*The way these formulas are changed will be more fully ex- 
plained at the end of the lesson. 
fCurves and their use will be explained at the end of this lesson. 



204 ELEMENTARY ELECTRICITY 

If a man can do a job in 6 days he can do 1/6 of it in 
one day. 

The part he can do in one day is the reciprocal of the 
number of days needed to do the whole job. 



Reluctivity = 



Permeability 

The formula he is using now becomes 

Flux T. 

Ampere turns = x 



1.25 a X permeability 

To make this formula more compact letters are used for 
the words. 

Different books use different letters, some even using 
German or Greek letters for the names. 

We will use A T for ampere turns. X (the last 
letter) for flux ; and p (the first letter) for permeability. 

The formula now looks like this : 

AT= X - L 



1.25 Xa-Xp 



This is a short way of saying: 

The ampere turns required to excite a magnet so as to 
produce a flux X are found in this way. 

( 1 ) Find the product of the flux and the length of the 
circuit. 

(2) Find the continued product of 1.25, the area, of 
the cross section of the circuit, and the permeability of 
the material. 

(3) Divide the first product by the continued product. 
By fussing with this formula the designer has obtained 

a knowledge of magnetism that can be obtained in no 
other way, but about this point he is apt to be disgusted 






DESIGNING CIRCUITS 205 

with the formula. He now on looking up the permeabil- 
ity tables or curves finds that he must calculate the dens- 
ity i. e. flux divided by area before he can find the perme- 
ability he wants; for the permeability changes with the 
change in density. 

After doing this he can replace each letter in the 
formula by the proper number and calculate the ampere 
turns. 

This formula may be changed to appear as : 

Y __L25XaXpxAT 

x - l ; 

This form is useless as a quick and accurate means of 
calculation for you must know the answer before you 
start. This is evident because p cannot be obtained until 
density is known and density is unknown until the total 
flux is determined. 

The way to use this formula is to guess at an answer, 
use a value of p accordingly and if the answer comes out 
too far away from the guess, correct the value of p and 
solve again. 

These formulas are not used in designing. 

The designer proceeds as follows : 

Suppose he has a magnetic circuit of type shown in 
Fig. 102. Y is the yoke which measures 12 inches be- 
tween centers of holes into which the poles P. are set. 
The path of the flux through the poles is 14 inches long 
in each. 

The part marked a is of sheet iron (annealed wrought 
iron sheets). The wires w are of copper. Between the 
armature iron and the pole piece on either side is an air 
gap of half an inch. 



206 



ELEMENTARY ELECTRICITY 



The magnetic circuit is through 2 X 14 = 28 plus 
12 = 40 inches of steel casting. Through 10 inches of 
sheet iron, and one inch of air gap. 

The pole pieces are, roughly, square 6x6 inches. The 
parts of P inside the coils or bobbins B are circular, 5 
inches in diameter. The yoke is a slab 5x12 inches. Arma- 
ture is 7 inches long and 7 inches wide. 




Pig. 102. The Magnetic Circuit of a Small Motor. 

The flux required is 1,800,000 lines. How many am- 
pere turns must the bobbins B contain ? 

From the laboratory connected with the factory he has 
obtained a table as is given in Lesson 13. Ampere Turns 
Required per Inch for Different Densities. 
He figures the densities : 

Yoke: 12x5 = 60 sq. in. 
1,800,000 -=-60 = 30,000 lines. 
Poles : 5 inch circles = 19.6 sq. in. 
1,800,000 -f- 20 = 90,000 lines. 
Air gaps : 6x6 = 36 sq. in. 
1,800,000— ^- 36 = 50,000 lines. 
Armature : yxy = 49 sq. in. 

Say 50 sq. in. 
1,800,000 ~ 50 = 36,000 lines. 



DESIGNING CIRCUITS 207 

Looking at the table he finds steel castings at density 
of 30,000 lines take 6.6 A T per inch; at 90,000 lines 
they require 57 A T per inch. 

Air at a density of 50,000 requires 15,665 A T per 
inch ; while sheet iron at 36,000 density needs 6.7 AT 
per inch. 

Referring back to the lengths of the circuits and tabu- 
lating the data he has : 

Material. Density. A T Length. Total A T 

per inch. required. 

Steel. 30,000 6.6 12 792 

Steel. 90,000 57 28 1,596 

Air. 50,000 15,665 1 15,665 

Iron. 36,000 6.y 10 6y 

Grand total 18,120 

Since there are two coils there will be 9,060 A T on 
each leg of the magnetic circuit. 

With a current of 1^ amperes 6,040 turns of wire will 
be required. 

In a similar manner when a designer wishes to know 
the flux that a magnet will produce, he measures its area 
and length. He then figures from the number of turns in 
the coil and the current he intends it to carry what the 
ampere turns are. Then he figures the ampere turns per 
inch and looking up in the table finds the density induced. 
Multiplying this by the area gives him the flux. 

This latter calculation can only be made when the cir- 
cuit is of one material and of the same size throughout, 
but one makes this, calculation only once to a thousand 
calculations of the ampere turns required. 



208 ELEMENTARY ELECTRICITY 

In cases where the circuit is of varying dimensions an. 
materials the ampere turns must be apportioned to each 
part of the circuit before the figuring is done. 

Question I. Suppose two pieces of Bessemer rod* 
each a foot long, one i% inch the other 3^ inches in 
diameter, were each to be magnetized to a density of 
77,500 lines (per square inch). How many ampere 
turns would each need? 

Answer. Table says 30 A. T. per inch for density of 
77,500 for soft steel. One foot is twelve inches. 
12X30=360 A. T. 

Question 2. But both will not require the same num- 
ber of ampere turns ? One is 1 sq. in. in area, the other 
is II sq. in. 

Answer. They both need the same magnetizing force 
because the lengths and densities are the same. 

Question 3. But if 360 A. T. are placed in each rod 
there will be a flux of 1X77,500= 77,500 in one, and 
11X77,500=852,500 in the other. How can this be? 

Answer. Because 360 A. T. on a 1 sq. in. rod pro- 
duces 77,500 lines, but the 3^4 diameter rod having 11 
times the area of the smaller rod offers only one-eleventh 
the reluctance and hence the flux is eleven times as great 
or 852,500 lines. 

But as flux is 11 times as great and area 11 times 
larger the density is the same. 

Question 4. Can you explain this in a different way? 

Answer. Suppose 36 turns of wire are made in one 
end of a long piece ; making the turns around a piece of 
1 ^8 -inch round wood, and a 10 ampere current is sent 

* Bessemer rod is rolled from the same material that steel 
castings are poured. They are almost like iron. When purchased 
they have a plating of copper on them to prevent rusting. 



RULE FOR DMftSifV &5§ 

through the whole wire. Suppose the turns are pulled 
apart until they are evenly spaced and the first and last 
turns 12 inches apart. This solenoid has now 
10X36-^-12=30 A. T. per inch. This will produce a 
density of 98 lines, say 100, in the air inside the solenoid. 

Suppose now the whole wire is used to make the 36 
turns. Keep the spacing the same but let the turns 
be nearly 10 inches across, from side to side. There will 
be an area of 78.5 sq. in. now under the influence of 
the solenoid. 

Each square inch is just as powerfully excited as be- 
fore, since there are 30 A. T. per inch surrounding it. 
Each of the square inches in the whole 78.5 will have 
100 lines induced in it. 

Question 5. Can you express this result as a rule? 

Answer. No matter what the size of a core the flux 
per square inch (density) depends on the number of 
ampere turns on each inch of its length. 



FORMULAS 



An expression 



__ Magnetizing force , v 

Flux = %-^ (I) 

Reluctance ' 

when written 

is called an algebraic formula. 

These formulas are capable of assuming different 
forms, but all these forms are brought about in a regular 
manner. 

Rules may be given to teach one how to make the 
changes but the principle underlying the rule should be 
understood. 

To place a term such as Z on the other side of the equal 
sign, remember that it must be moved either up or down. 
If in the denominator on one side it must move to the 
numerator on the other side. If in the numerator on one 
side it must be placed in the denominator on the other. 
Whole numbers are to be considered as in a numerator. 

In (2) let us place Z on the left hand side of the equal 
sign. 

Z XX = ATX 1.25. 

The reason for this rule is : Formula (2) must be con- 
sidered as an equation. The left side is equal to the right 
side. Nothing must be done which will disturb this 

ecualirv. 

210 



FORMULAS 211 

If we multiply both sides of (2) by Z, the equality will 
not be affected, and the equation will look like 

zxx= ZXATXi. 2 5 ( 4 ) 

But the two Z's on the right and side cancel, and we 
have 

ZXX-ATX1.25. (3) 

Suppose that in (2) we wished to change the formula 
so that the term A T would stand alone. 

Z comes to the left numerator; 1.25 comes to the right 
denominator, and we have 

^i=AT (5) 

1.25 KDJ 

Since these things are equal it makes no difference 
which is written first, so we write 

AT=^^ (6) 

1.25 v ' 

To substitute for Z any other letter or expression, we 
must first be sure that the thing we wish to substitute is 
exactly equal to Z. 

From the Lesson we know 

Substitute in (6) for Z its value as given in (7) by 
writing in the space occupied by Z the other expression : 

AT=^P (8) 

1.25 v > 

The right hand side is now a complex fraction which 
must be simplified. 



212 



ELEMENTARY ELECTRICITY 



Do so in this way : Copy the numerator of the Corn- 1 
plex fraction down separately; 

I 



XX 



(9) 



aXp 

Multiply the fraction by the whole number X in the 
ordinary way of arithmetic : 

a x p v ' 

Go back to the complex fraction and copy its denom- 
inator changing it to a fraction. 

f <"» 

Now draw a heavy line; place (io) abov£ it and (ii) 
below it. 

ixx 



aXp 
i-25 



(12) 



: Multiply the extreme top and bottom of (12) for a 
numerator, and. multiply together the middle parts for a 
denominator, as shown by the brackets. This gives 

IXX , . 

(13) 



(14) 





a 


X P X 1.25 


which we can 


put back 


in (8) and get 




AT- 


ixx 




. a X p X 1.25 


Rearranging 


r the letters we get 




AT = 


xxx 



1,25 X a X p 



as we did in the Lesson. 



(15) 

- - " 



CURVES. 

The use of curves in engineering work was started 
with an idea of showing results quickly and making them 
easily understood. 

Suppose you were trying to impress on a man's mind 
the fact that the traffic on suburban trains varied in a 
regular manner every morning and evening, and that 
the through trains were evenly loaded all day. Also that 
the weight of the passengers in the through trains aver- 
aged 5% of the weight of the train, while the weight of 
suburban passengers varied from 2% to 20% of the 
train's weight, according to the time of day. 

Hand him the following table and while the informa- 
tion is there it will take him some time to get it into his 
head. Ask him suddenly : "At what times are the sub- 
urban and through trains equally loaded with passen- 
gers?" 

See how long before he finds the information which 
will enable him to answer. 

Table of the Percentage of Passenger Weight to Light 

Train Weight, Grand Central Station to Mott 

Haven Junction, New York. 

SUBURBAN TRAINS. 

Time. • P f r - Time. Per " 

centage. centage 

Midnight 2. 6 12. 

1 2.25 7 18. 

2 2.75 8 20. 

3 3- 9 18.5 

4 3-5 10 14.5 

5 5- 11 9- 

213 



214 



ELEMENTARY ELECTRTCTTY 



Time. 

Noon 
I . . 



Per- 
centage. 

8.2 5 

8-5 

8.75 

9-5 

. 11. 

. ... 16. 

Midnight . 



Time. 

6 . 

7 • 

8 . 

9 • 

10 . 

11 . 



Per- 
centage 

..19. 

..19.25 

..16. 

. . IO.5 

.. 6.5 

.. 4=25 



2. 



THROUGH TRAINSo 

Five per cent at all hours 
Now hand him the following curve (Fig. 103) 



" - 7^ 


-,*V 


» f \ 


7 \ 


7 - -4 4 


f 4 


I 4 4 


^ it 


5 4 -t^ 


t 


K t U 


1 t 


a t f 


I I 


-S I 


f 3 


C -j 4^ 


ub l urban 


Sc 4 4 -J 


Trains / 


,_IO -+ -J 


-A-/~ \- 


*! t * 


-" X 


7 t 


\ 


* 




1 ji r, ""i 


?fyo/7 fr*dins \ 


4 4 4- 




3 ^-^ 


v 




3 












12 1 2 3 4 5 6 7 e 3 10 11 KL r 2 3 i 5 4 7 4 9 10 lU 
Midntqht A.M. Noon P.M. MidniqN 



Fig. 103. Percentage of Passenger Weight to Total Weight of Train. 



DRAWING CURVES 215 

How quickly he grasps the way in which the traffic 
varies. He notices much more quickly than he would 
by use of the table that from 5 to 8 in the morning there 
is an abrupt rise in traffic, while from 6:30 to 11 in the 
evening there is an equal decline in traffic at a much 
slower rate. He notices the great changes in train 
loads between 5 and 7 in the morning and the slight 
change between 2 and 4 in the afternoon. 

Ask him again the question, and see how quickly it is 
answered. 

A curve is certainly a great thing for imparting in- 
formation. 

The curve is drawn from the table in the following 
way: Procure a piece of paper printed with lines run- 
ning across at right angles in both directions and at 
some convenient distance apart, say 1/10 of an inch, or 
for finer work, 1 millimeter.* This is called cross-section 
paper. 

Determine which of the set of two numbers you are 
most anxious to have show up strikingly, and number 
each line up along the left edge accordingly. 

Each space can be 1%, but if the percentages run up 
very high, you might have to call each space 5%. 

Number the lines along the bottom of the sheet ac- 
cording to hours. Let one space represent 15 minutes 
or one-quarter of an hour, or if there is not room for 
this, let each space count one hour. When the lines are 
properly numbered, lay out the curve. 

Place your pencil on the first vertical line (marked 
midnight). Run pencil up along this line until you reach 



inch 



* A millimeter is 1/10 of a centimeter, i. e. about 0.04 of an 
h 



216 



ELEMENTARY ELECTRICITY 

Make a dot where 



the horizontal line marked 2% 
these two lines meet. 

On the next line, marked I a. m., run the pencil up 
till opposite a point one quarter the way up between 
the 2 and 3% lines. This represents 2.25. Make a 
dot here. 



PERMEABILITY CURVES FOR 



STEEL CASTINGS 
ORDINARY CAST IRON 



on 

< 

or 
co 

Id 

0. 
CO 

Id 
Z 



>- 
CO 



H€69& 



H90^ 



*0£ 



£>>>> 



m- 



t4} 



00 



00 



€€0 



*» 



00 



**** 



oe 



^> 



00 



0B 



SM 






132 



CASTINGS 



tens 



O 100 200 300 400 500 600 700 800 900 

MAGNETIZING FORCE:— ampere-turns per inch length 

Fig. 104. Permeability Curves. 

With the figures from the table in this way make a 
dot at the point where the vertical time line cuts the 
horizontal per cent line. 

There will be one dot for every set of figures in the 
table. When all dots are placed, draw a line through 
them and you have the desired curve. 



PERMEABILITY CURVE 217 

Figure 104 contains two curves drawn from data ob- 
tained from the table of ampere turns per inch at dif- 
ferent densities. 

Certain information can be more easily gained and is 
shown in a more impressive manner by these curves than 
by the table. 

Notice that as you steadily increase the A. T. per inch 
on a steel casting at first the result is a great increase in 
flux, and the density rapidly increases, but after a while 
the density increases very slowly. 

With cast iron this same statement is true, but the 
effect is not so marked. At first you get a rapid increase 
in density and later a slower increase. 

The first and last parts of the steel curves are prac- 
tically straight lines. Designers often speak of the 
"straight line portion of the curve." The part between 
these two is called the "knee" of the curve. 

The cast iron curve has no definite straight line por- 
tions and the knee is so large that it is difficult to ex- 
actly locate it 



LESSOX 15. 
Primary Batteries. 

There are many places where we need a small amount 
of electrical power, so little that running a line to the 
point would not pay. In such cases we uses batteries. 

A storage cell needs more attention than a primary 
cell. so many automatic signals, call bells, and such are 
operated by primary cells. 

There is a deal of truth in the statement. "There is 
electricity- in everything." The hard job is to get it out. 

Suppose you throw a dozen shovelfuls of fine damp 
coal into the firebox, and forget to close the door. The 
result you get is not the fault of the coal. It was full 
of B. T. U.'s 1 and you gave them a chance to get out, 
but not in the proper manner, and they failed to do the 
work of making steam. 

"What a difference there is if this same kind of coal 
in larger pieces is fired two shovelfuls at a time with 
just the right quantity of air. 

So it is with electricity ; we must treat our materials 
in exactly the correct manner if we expect a production 
of current worth the money expended. 

If one pound of zinc be placed in dilute sulphuric acid- 

1. B. T. U. is an abbreviation for a British Thermal Unit, 
being the amount of heat required to raise one pound of water 
from 39 to 40 degrees Fahrenheit (ordinary thermometer). One 
pound of good coal contains 1.200 B. T. U. 

2. Buy oil of vitriol and dilute by pouring the acid slowly 
into 20 times as much water, stirring with a piece of glass. An 
earthen ware pot is the safest thing to mix in as the great heat 
generated will not crack it. 

218 






SIMPLE CELL 219 

it will dissolve and give out 1,026 B. T. U., but no elec- 
tricity. You will notice that there are bubbles of hydro- 
gen gas coming up from the zinc and a 'thermometer 
would show the increase in temperature. A certain part 
of the energy liberated might have been obtained as 
electricity if the zinc were treated in the following way : 

Placing the zinc and a piece of copper in a jar of 
dilute sulphuric acid, not allowing them to touch below 
the liquid, allowing them to touch above it, or connect 
them with a copper or iron wire. Now the thermometer 
will rise very little, electricity will flow through the 
wire and the wire exhibit magnetic qualities. We agree 
to say that the current flows from the copper to the 
zinc outside of the cell and from the zinc to the copper 
in the liquid. 

We have now started into action the simplest of the 
primary batteries. 

This same experiment made with a strong solution of 
common salt in water will work as well. 

If you attempt to use the current from such a simple 
cell you will find that it is very quickly apparently ex- 
hausted. 

To be a commercial success a cell should deliver cur- 
rent more or less continuously until its zinc is all con- 
sumed. 

In the words "more or less continuously" lies the dis- 
tinction made between cells. An "all around service" 
cell is difficult to design, so that we have Open circuit. 
Semi-closed and Closed circuit types of cells. 

The main thing in any cell is to avoid the tendency of 
the cell to "lay down" while there are yet plenty of 
chemicals in the cell capable of, under proper circum- 
stances, delivering electricity. 



220 ELEMENTARY ELECTRICITY 

This stoppage of the cell's activity is called Polariza- 
tion. 

Return to the simple cell of copper, zinc and sulphuric 
acid. If it has been used long enough to "lay down," 
examine the plates. The copper one is entirely covered 
with bubbles. These are the cause of the cell's non- 
action. The cell is polarized. The reason for this name 
and the cause of the non-action will be best understood 
by first learning this table: 

TABLE OF VOLTAIC CELL MATERIALS. 

Direction of current through the wire in external cir- 
cuit. 



Positive 


Z 


I 


L 


C 


S 


C 


Negative 




i 


r 


e 


o 


i 


a 




Plates 


n 


o 


a 


P 


1 


r 


Plates 




c 


n 


d 


P 
e 

r 


V 

e 

r 


b 



n 





Direction of current through solution in cell. 8@* 

This table is the result of experiments such as you can 
perform yourself. In another glass of dilute sulphuric 
acid place two pieces of zinc and connect them. They 
dissolve but no electricity is delivered. Try two iron 
plates with the same result, with perhaps less corrosion 
of the metal by the acid. Two pieces of lead give no elec- 
tricity and are hardly affected by the acid, while two 
sticks of carbon are not even attacked by the acid. With 
zinc and iron you get a weak and almost useless cell, with 
current flowing from the iron to the zinc. Zinc and cop- 
per we know to be good, but we find that zinc and carbon 
is better. 



POLARIZATION 



221 



This table is evidently arranged so that the further 
apart the metals stand in it the better a cell they make. 

This should set you thinking. A zinc plate and a bub- 
ble covered copper plate will not make a cell. Therefore 
bubbles must make a positive plate. 



posjtjve 
Pole on 

electrode 



ATECATJVE 
PLATE 



HBCATIVB 
P01£ OR 
ELECTROOR 




Fig. 105. Names of Parts of Cell. 



You must find now what the bubbles are and how 
they act electrically. 

The things in the cells are : 

Zinc : that is zinc and impurities. 

Copper: that is copper, pure. 

Sulphuric Acid : that is sulphur, hydrogen and oxygen. 

Water: that is hydrogen and oxygen. 

Since the bubbles are of gas they must be either hydro- 
gen or oxygen. 

Chemistry books will tell you that oxygen makes a 
fine negative plate and hydrogen a fine positive plate; 
just about as good as zinc. 



222 ELEMENTARY ELECTRICITY 

The cell has then two positive plates in it, and has twj 
negative poles. (See Fig. 105.) The cell is polarized. 

We have learned so far that a cell must have two dif- 
ferent materials immersed in a solution and the more 
rapidly it attacks one and the less it affects the other 
the better the cell. For this reason zinc and carbon, be- 
ing cheap commercial products, are almost universally 
used in primary cells. 

Also a cell to be a commercial success must either 
not produce hydrogen gas or get rid of it after pro- 
duction. 

We will now describe some of the most used cells, 
classifying them under headings as follows : 

Open circuit: A cell designed for intermittent work. 
Periods of work short, intervals of rest long. Usually 
designed for small currents. When not in use these cells 
must be left on open circuit. 

Semi-closed: A cell designed for fairly steady work. 
Periods of work long, intervals of rest short. Often de- 
signed to produce heavy currents. When not in use 
these cells must be left on open circuit. 

Closed circuit: A cell designed for continuous work. 
Periods of work long, intervals of rest very short. Usually 
designed for very small currents. Almost impossible to 
design so as to produce much current. When not in use 
they must be left on closed circuit. 

Polarization prevented: Cell so. designed that no 
hydrogen gas is produced by chemical action of cell. 

Polarization cured: Cell produces hydrogen, but a 
chemical placed in the cell turns the hydrogen to water, 
which is harmless. 

Polarization delayed: Cell has very large and ab- 
sorbent negative plate. 






DESCRIPTIONS 



223 



CELLS COMMONLY USED IN RAILROAD WORK. 

The Carbon Cylinder Cell. These are sold under the 
name of Law, Samson, Hercules, etc. It is an open cir- 
cuit, polarization delayed type. They give a pressure of 
1.5 volts and have a resistance of I to 2 ohms. Two of 
them are shown in Fig. 106. 

The carbon element is made with as large a surface 
as possible. Carbon and charcoal have a remarkable 
power of absorbing gases. A cubic inch of charcoal will 
condense and absorb 20 to 30 cubic inches of gas. 




■.- u ■ ■ ■. • ■ 
Fig. 106. Carbon Cylinder Cell. 



The zinc element is a rod and the fluid a strong solu- 
tion of sal ammoniac in water. The scientific name of 
this chemical is ammonium chloride. 

The action of the cell dissolves the zinc, forming zinc 
chloride, which dissolves in the water. A little am- 
monia and hydrogen gases are set free. The ammonia 
is dissolved by the water and the hydrogen absorbed by 
the carbon. 



224 



ELEMENTARY ELECTRICITY 



In time the carbon gets soaked full of hydrogen, and 
to restore the cell it should be taken out and boiled in 
water for an hour. 

These should only be used for call bells in offices or 
such unimportant work. 

Leclanche Cell. This is an open-circuit., polarization 
cured type. They are made in several forms. Voltage 
1.5 and resistance I to 4 ohms. Uses sal ammoniac, zinc 
and carbon. 




Fig. 107. Carbon Cylinder Cell with. Depolarizer. 



The carbon cylinder cell is sometimes modified to the 
Leclanche type by making the carbon element with a 
bottom and no opening in the sides. This carbon can or 
bucket is filled with lumps of black oxide of manganese 
(manganese dioxide^. The zinc is made in a cylindrical 
form, surrounding the carbon. This cell is shown in 
Fig. 107. 

The hydrogen is absorbed by the carbon but the man- 
ganese dioxide, being in contact with the carbon, gives 
up half of its oxygen to the hydrogen forming water, 
while it is reduced to manganese monoxide. 

T .is cell is useful for call bell work, operating mag- 



6PEN CIRCUIT CELLS 



225 



nets on interlocking machines, running tell-tales on inter- 
locking boards and such other intermittent light work. 
There is an older form of Leclanche cell shown in Fig. 
108, where the carbon is placed in a cup of unglazed 
earthen ware (like a yellow flower pot) called a porous 





Fig. 108. Ordinary Leclanche Cell. 



Fig. 109. Elements of the 
Gonda-Leclanche Cell. 



cup. The manganese is packed around the carbon slab. 
This form does not give such a large current as the 
cell in Fig. 107 because its resistance is high, often as 
much as four or five ohms. 

A much used, form of the Leclanche cell is the Gonda 
cell. The elements are shown in Fig. 109. 

Here the manganese is powdered, mixed with cheap 
molasses, then bv heat a,nd pressure formed into slabs. 



226 ELEMENTARY ELECTRICITY 



These are attached to the carbon plates by rubber bands. 

The bother and resistance of the porous cup is avoided. 

The usual charge of a Leclanche type cell is a generous 
quarter pound of sal ammoniac dissolved in sufficient 
water to fill the jar two-thirds full after elements are in 
place. 







Fig. 110. Elements of Gravity Cell and Jar. 



The Gravity Cell. This is a closed circuit cell with 
polarization prevented. It is very much used for tele- 
graph circuits, operating the electrical devices in the 
lock and block signals, the motors in automatic signals 
and generally around interlocking plants/ Its pressure 
is i volt and its current capacity rather low for its re- 
sistance is 3 or 4 ohms. 



CLOSED CIRCUIT CELLS 



22? 



This cell is made in many forms called Bluestone cell, 
crow-foot battery, Lockwood cell, etc. 

The parts of a gravity cell are shown in Fig. no, and 
the assembled cell in Fig. in. 




Fig. 111. Gravity Cell Ready for Use. 



The glass jars should be about 7 inches high and 6 
inches in diameter. The zinc is cast in a shape so as to 
be easily suspended from the edge of the jar. The form 
shown is called a crow-foot zinc. It weighs about 3 
pounds. 

The copper element shown on left of Fig. no is made 
of three sheets riveted together at center and then spread 
out as shown. The rubber covered wire must be at- 
tached to the copper element by riveting. If soldered 
the joint would be eaten away by electrical action. 



228 ELEMENTARY ELECTRICITY 

To set up a cell of ordinary size which holds about 0.8 
gallons of liquid make two solutions, one of copper, the 
other of zinc. 

Zinc solution: Pint and a half of pure soft water 
and 10 oz. of crystallized sulphate of zinc (white vitriol). 
Mix until dissolved and let it stand half a day in a glass 
jar. 

Copper solution : Two and a half pints of soft water, 
4 ozs. of crystallized sulphate of zinc, 8 ozs. crystallized 
sulphate of copper (blue vitriol). Mix and let stand a 
few hours in a glass jar. 

Dip edge of battery jar for an inch in melted paraffin 
and let it cool. 

Place the parts in jar as in Fig. in and pour jar nearly 
three-fourths full of the zinc solution. Place it at once 
in the spot where it is ro be used and pour in the copper 
solution. 

Insert a glass funnel in the top of a piece of ^-inch 
rubber tubing. Hold funnel so that lower end of the tube 
will be in the middle of the jar and just a little above 
the bottom. 

Pour in the copper solution slowly until the copper 
element is completely covered. Place the cell into service 
immediately. 

This cell will show a sharply defined line between the 
blue copper solution and the colorless zinc solution. This 
separation of solutions is essential to the cell's health. 
Leaving the circuit open for any length of time will allow 
the solutions to mix and spoil the cell. 

The action of the cell is such that no hydrogen is per- 
manently formed. The zinc is steadily dissolved into 
the zinc solution, setting free some hydrogen. This 
forms with the copper sulphate, sulphuric acid and me- 



GRAVITY CELL 



223 



tallic copper. The sulphuric acid dissolves more zinc, 
while the copper plates itself on the copper element at the 
bottom of jar: 

The zinc is consumed and the copper plate grows 
larger. 

The effect of continued action is to increase the 
strength of the zinc solution so that it tends to settle to 
bottom of jar. 



r\ 




Fig. 112. Long Service Copper Element for Gravity Cell. 



The copper being taken out, bit by bit, from the copper 
solution this latter gets lighter in weight and tends to 
rise, being pushed up by the zinc solution. 

If the blue solution of copper sulphate ever touches 
the zinc it will copper plate it at once. The cell will then 
have two copper elements and stop working. 

Cells should be given some attention, and clever man- 
agement will keep a gravity cell working continuously 
for an almost indefinite time. 



230 



ELEMENTARY ELECTRICITY 



As helps in the maintenance of cells two improvements 
have been made. 




Pig. 113. d'Infrevilles Wasteless Zinc. 

The form of copper element shown in Fig. 112 is bet- 
ter when heavy currents are not needed. It is a copper 
ribbon 4 feet long and J4 an inch wide, coiled like a 







Fig. 114. Using Up Old Zincs. 



clock spring. Zincs shaped like Fig. 113 are used until, 
the prongs are all eaten off. A new one is then put in 



HYDROMETER 231 

service and the old one jammed into the bottom of the 
new one as shown in Fig. 114. 

These zincs are hung from a spring clip shown in Fig. 
115, which lays across the top of the jar. The stud on 
the zinc makes a tight friction fit with the hole in the 
hanger, due to the springiness of the metal. 

To keep cells in order a hard rubber syringe with the 
nozzle at right angles to barrel, holding about a pint, and 
a hydrometer should be obtained. 




Fig. 115. 



The hydrometer (Fig. 116) is a hollow glass float 
loaded with shot so as to float upright. The heavier a 
liquid the more of the stem sticks up above the surface. 

These hydrometers are graduated on stem in actual 
specific gravities or in degrees Baume (pronounced 
Bomay). One with a stem about two inches long gradu- 
ated from 15 to 40 Baume, or from 1.11 to 1.40 specific 
gravity, is best for battery work. 

The first signs of exhaustion in the cell will be a fading 
of the deep blue color of the copper solution and a low- 
ering of the line of separation between blue and white 
liquids. 

When this occurs drop in about an ounce of copper 
sulphate in lumps. Be sure the lumps fall to the bottom. 



232 



ELEMENTARY ELECTRICITY 






There will always be a lot of fine powder at the bottom 
of the copper sulphate barrel. Use this for making up 
new cells when possible. If too much accumulates for 
this purpose, make a saturated solution of it in water. 




Fig. 116. Hydrometer with Baume Scale. 



A saturated solution is one where the water has dis- 
solved all it possibly can of the chemical and leaves some 
yet undissolved on bottom of jar after repeated stirring. 

Place this in cells showing signs of exhaustion in same 






GRAVITY CELL 233 

way as the copper solution was placed in a newly set up 
cell. 

The zinc solution should be tested as frequently as 
possible. Once in two weeks is not too often. Drop the 
hydrometer gently in. Should it read 115 draw some out 
with syringe and replace by fresh water. 

Do not let it go below 1.10. If you have a Baume 
scale these numbers are 20 and 15 degrees. Throw all 
the removed zinc in a wooden tub, whether from working 
cells or from old cells, to be renewed. 

Keep half a dozen pieces of metallic zinc in this tub. 
Any copper in this solution, mixed by cell's action, will 
turn to a reddish brown curd which can be filtered out. 
Reduce the clear liquid to 1. 10 and use in making up new 
cells. 

Watch your zinc. Should any brown hangers develop 
on it, detach them with a bent wire and let them fall to 
bottom of cell. 

In time, in spite of all care, the zinc in a cell gets 
reddish brown all over. It is now time to give a com- 
plete overhauling. 

Take the cell out of service. Syphon off zinc solution 
into the tub. Lift zinc out carefully and at once scrub 
clean with a wire brush. Wash and replace in another 
cell at once or dry thoroughly and keep dry until needed. 

Syphon off the rest of the liquid into another wooden 
tub and use after filtering as copper solution to make up 
new cells. 

Any lumps of copper sulphate in the bottom take out, 
rinse and put in other cells. 

The mud in bottom of cells and in the zinc solution 
tub should be dried- and sold to brass founders as "bat- 
tery mud." 



234 



ELEMENTARY ELECTRICITY 



The copper plates taken from cells should be kept 
completely covered with water, wire and all, until needed 
again. 

When they get too heavy and cumbersome sell them, 
as they are an especially pure form of copper. 

Never leave gravity cell on open circuit; the liquids 
will mix. 







Fig. 117. Fuller Cell. 



The Fuller Cell. Semi-closed circuit type, for heavy 
duty. Long periods of work with little rest. 

Polarization cured. Pressure 2 volts, resistance 0.5 
ohms. Cell shown in Fig. 117. 

These cells are carbon and zinc, and since the chemical 
which converts the hydrogen to water will attack the 
zinc, a porous cup is used. 

The carbon or the zinc can be placed in the porous 
cup, but the zinc usually is. A tablespoonful of mercury 
is placed in bottom of porous cup, the zinc set in and 
the cup filled with very dilute sulphuric acid (1 acid, 50 
water). The carbon is then placed in the outer jar, the 



EDISON CELL 235 

porous cup being also in, and the outer jar filled three- 
quarters full of battery fluid or electropoin. 

This is composed of 4 ozs. of bichromate of soda, 1 54 
pints of boiling water, mixed and cooled; then while 
slowly stirring add little by little 3 ozs. sulphuric acid 
taken out of a carbon (not diluted). NEVER POUR 
WATER INTO ACID. 

The bichromate of soda has so much oxygen in it that 
it will turn the hydrogen to water, changing itself to 
chromate of soda. 

When the interior of the porous cup gets dark green 
colored a cup should be soaked in 1 to 50 acid for an 
hour and then mercury placed in bottom and zinc set in. 
Simply take out old cup and insert new one in its place. 

The old zinc should be cleaned, porous cup washed 
and then boiled in water and both placed in stock. 

These cells should be left on open circuit when not in 
use. They are very powerful, but nasty to handle and 
not as cheap as the gravity cell. When the electropoin 
gets greenish it soon becomes exhausted, then throw it 
away. Cold battery rooms or pits affect this cell less than 
the gravity cell. 

Edison-Lalande Cell. This is a semi-closed type with 
polarization cured. It has a resistance of 0.2 ohms and 
a very low voltage, 0.7, but is a bull dog for holding on. 
It will, when set up, start in to deliver a heavy current 
and keep at it until all its chemicals are used up. It 
needs no attention and is built so that you can not 
give any. 

When it stops take out the copper and sell it, throwing 
everything else out. Clean up the jar and fit out again. 

The cell uses zinc and oxide of copper plates immersed 
in a solution of caustic potash. The oxide plate is shown 



236 



ELEMENTARY ELECTRICITY 



in Fig. 118 and the complete cell with a glass jar in Fig. 
119. Porcelain jars are usually furnished. 

The caustic potash comes in sticks sealed up in a tin 
can. 

Place the elements in jar and fill with water to about 
one inch of the top. Take out the elements and put in 
the sticks of potash. 




Pig. 118. Oxide Plate of Edison-Lalande Cell. 



Stir constantly while dissolving, for it gets very hot 
and might crack the jar. Be very careful not to get 
caustic potash on your flesh. It not only burns ter- 
ribly, but makes a wound which is very hard to heal. 

If you buy potash by bulk, make the solution up to 
1.33 on specific gravity scale or 38 on the Baume 
scale. 



CAUSTIC POTASH CELL 



237 



Place the zinc and copper oxide elements in the jar, 
seeing that they are properly separated by the hard 
rubber buffers. Pour the bottle of oil over the top of 
solution and place cover on. 




Fig. 119. Edison-Lalande Cell. 



If buying oil by bulk, get a heavy paraffin oil which 
will read 1.46 specific gravity on 48 Baume and pour 
a l /\ inch layer on each cell. 

These are good cells, but any sulphuric acid or caus- 
tic potash cell is a nasty thing to handle. 

The action of the cell dissolves the zinc, setting free 
hydrogen, which is changed to water by the copper 



238 



ELEMENTARY ELECTRICITY 



oxide, which is reduced to pure copper by giving up the 
oxygen in it. 

The Dry Cell. Shown in Fig. 120 is really a moist 
cell sealed up water tight with cement or glue. 

The can is made of zinc and serves as one element, 
while a carbon plate or rod is the other. Around the 
carbon is packed a mixture of powdered manganese 
dioxide, carbon and flour., while the rest of the can is 




Fig. 120. Dry Cell. 



filled with a mixture of plaster, oxide of zinc and flour; 
the whole being soaked with a solution of sal ammoniac 
and zinc chloride. Pressure 1.4 volts. 

These are very useful for testing, as they can be 
carried around in a satchel or your overcoat pockets. 

Whenever they are used in sets see that their rest- 
ing place is dry, otherwise the moisture will connect all 
the zinc cans together and cause them to run down. 

The principles on which primary cells work are sim- 
ple and well understood by most people ; yet there are 
men trying yet to design a cell to do more than is 
possible. 



LOCAL ACTION 239 

The best that could be done with a cell using zinc 
as the dissolved metal is to get one horse-power-hour* 
per pound of zinc. 

There are inevitable wastes which prevent us doing 
as well as that, so with coal at % cent a pound and zinc 
at 1 6 cents it is evident that the primary battery will 
only be used when circumstances force us to use it. 

One great waste is Local Action: 

Local Action. Commercial zinc or spelter contains 
small particles of carbon and iron which with the zinc 
they are imbedded in form small local cells producing 
electricity where it cannot be gotten at for use, and the 
zinc is continuously dissolved whether the cell is on 
open or closed circuit. In the sal ammoniac batteries 
sometimes the change in the strength of the solution 
will cause the zinc to be eaten through at or very near 
the surface of the solution. 

The remedy for this is 

Amalgamation. Mercury forms a soft paste or amal- 
gam with all the metals except iron, and will not dis- 
solve carbon. Advantage is taken of this fact and local 
action is prevented by cleaning the zinc with sand paper, 
washing with dilute sulphuric acid, and while wet rub- 
bing on mercury (quicksilver) with an old brush or a 
rag tied to a stick. N. B. Mercury is a poison. The 
zinc becomes bright, covered with a layer of zinc- 
mercury amalgam; and the particles of iron and carbon 
are merely covered up and protected from the acid, 
which cannot corrode the mercury. During the action 



* An horse-power hour is the work done by a one horse- 
power engine running at full load for an hour; or the work 
done by a 10 H. P. engine running at half load for one-fifth' of 
an hour, etc. 



24C ELEMENTARY ELECTRICITY 

of the cell the zinc dissolves out, and the mercury eats 
its way into the zinc, reforming the amalgam. When 
the zinc around the particles is eaten away they fall out 
to the bottom of the battery jar and do no harm. 
Zincs for batteries are sometimes cast with 5% of mer- 
cury in them. When the zinc is in a porous cup it is a 
good thing to pour a tablespoonful of mercury into the 
cup and then set the amalgamated zinc in. With all the 
precautions that can be taken about 3% of the zinc 
put in the cell is wasted in local action. 



CATECHISM TO LESSON 1 5. 

1. What is an open circuit cell? 

2. What is a closed circuit cell ? 

3. What is polarization? 

4. What means are used to prevent polarization? 

5. What ways are there of curing polarization? 

6. What two materials of ordinary cost make the 
best cell? 

7. How good a cell would zinc and iron in sal am- 
moniac make? 

8. Would the results of a zinc silver combination 
in sulphuric acid give results worth the cost? 

9. What is the name of the wet end of the zinc 
element? The dry end? 

10. What is the name of the wet end of the copper 
or carbon element? The dry end? 

11. Why would not a zinc-copper cell made like a 
Law cell operate? 

12. What cells would work well on a signal circuit 
closed 98% of the time? 



CATECHISM 241 

13. What cell would work weli on the motor of a 
signal, current closed 1 % of the time ? 

14. What is Local Action? 

15. What is amalgamation of zine? 

16. How are zincs amalgamated? 
iy. What is a hydrometer ? 






LESSON 16. 

Storage Batteries. 

The storage cell is rapidly pushing the primary bat- 
tery aside in signal and fire alarm work on account of : 
(i) Its high voltage. 

(2) Its great current capacity. 

(3) The lowering of total battery expense if used for 
several years. 

(4) Its steadiness of action. 

Storage cells are used in train lighting to furnish light 
when train is not in motion and to steady the supply of 
current. 

They are used in some cases to furnish the power to 
operate switches on locomotives and motor cars. 

In power houses they offer a reserve supply of power 
and act as a steadier of the load on the generators. 

The simplest storage cell would be two strips of lead 
immersed in dilute sulphuric acid. When current is sent 
through them one plate turns a dark brown color and 
the other a grey color. After an hour's passage of cur- 
rent reverse the connection and charge the other way. 
The plates will change color — the grey one becoming 
brown and the other one grey. 

If this charging first in one direction and then in the 
other be kept up, you will notice that after each reversal 
of the current through the cell the acid is quiet but soon 
begins to gas or boil. This is the signal to reverse the 
current as the cell is charged. 

242 



Storage batteries 



243 



When the cell takes several hours to gas it is in condi- 
tion to use. 

After one of the reversals continue to charge until cell 
has gassed about fifteen minutes. Remove the charging 
wires and connect to anything you wish to run. About 
70% of the power you put into the cell can now be taken 
out 




Fig. 121. Lead Grid. 



You may now use this as a storage cell, charging it up 
till it gasses and then using the accumulated electricity as 
you please. 

You always lose 30% but you have the advantages of 
portability and ability to work when engines are shut 
down. 

In time you will notice that the lead plates become 
spongy and should the cell be used long enough the plates 
will finally crumble and break. You will notice that the 
more spongy the plates become the greater a charge they 
are capable of holding. 

In fact, just before your battery goes to pieces its ca- 
pacity is the greatest. 



m 



ELEMENTARY ELECTRICITY 



To make a commercially practical cell we would pro- 
ceed thus: 

The lead plates would be replaced by grids as shown in 
Fig. 121 or by grooved plates as in Fig. 122. 

Litharge and sulphuric acid is mixed to a stiff paste 
and the grids or grooved plates plastered with the paste 
and stood up to dry. This makes a negative plate. 





Fig. 122. Grooved Lead Plates. 



Using a paste of red lead and sulphuric acid the posi- 
tive plates are formed in the same way. 

The objection to a storage cell using these plates is 
that after very little use they go to pieces. The changing 
of the red lead to the brown oxide, and the changing of 
the litharge to spongy lead is accompanied by a swelling 



STORAGE BATTERIES 



245 



and shrinking of the material. This loosens up the 
pasted mass and it begins to fall out. 

Most of the ingenuity of inventors has been concen^ 
trated on making plates which would hold the active rha^ 
terials firmly and continually. 

Perhaps one of the best lead-lead (i. e. lead for both 
plates) is the Electric Storage Battery Company's Chlor- 
ide Cell. 




Fig. 123. Chloride Accumulator. 



This cell is shown in Fig. 123. Its method of manufac- 
ture is interesting and is practically as follows : 

The first thing is to get finely divided lead which is 
made by directing a blast of air against a stream of the 
molten metal, producing a spray of lead which upon cool- 
ing falls as a powder. This powder is dissolved in nitric 
acid and precipitated* as lead chloride on the addition of 
*Turned back to a solid. 



246 ELEMENTARY ELECTRICITY 

hydrochloric acid. This chloride washed and dried forms 
the basis of the material which afterwards becomes active 
in the negative plate. The lead chloride is mixed with 
zinc chloride,, and melted in crucibles, then cast into small 
blocks or tablets about 24 mcn square and of the thickness 
of the negative plate, which according to the size of the 
battery varies from % inch to 5/16 inch. These tablets 
are then put in molds and held in place by pins, so that 
they clear each other 0.2 inch and are at the same dis- 
tance from the edges of the mold. Molten lead is then 
forced into the mold under about seventy-five pounds 
pressure, completely filling the space between the tab- 
lets. The result is a solid lead grid holding small 
squares of active material. The lead chloride is then 
reduced by stacking the plates in a tank containing a 
dilute solution of zinc chloride, slabs of zinc being al- 
ternated with them. The assemblage of plates consti- 
tutes a short-circuited cell, the lead chloride being re- 
duced to metallic lead. The plates are then thoroughly 
washed to remove all traces of zinc chloride. 

A later form of negative plate consists of a "pocketed" 
grid, the opening being filled with a litharge paste; this 
is then covered with perforated lead sheets, which are 
soldered to the grid. The positive plate is a firm grid, 
composed of lead alloyed with about 5% of antimony, 
about 7/16 inch thick, with circular holes 25/32 inch 
in diameter, staggered so that the nearest points are 
.2 inch apart. Corrugated lead ribbons 25/32 inch wide 
are then rolled into close spirals of 25/32 inch in diam- 
eter, which are forced into the circular holes of the 
plate. By electrochemical action these spirals are formed 
into active material, the process requiring about thirty 
hours ; at the same time the spirals expand so that they 



STORAGE BATTERIES 247 

fit still more closely in the grids. This form of posi- 
tive is known as the Manchester Plate. 

In setting up the cells the plates are separated from 
each other by special cherry wood partitions, the per- 
forations being connected by vertical grooves to facili- 
tate the rising of the gases. Sometimes glass rods are 
used as separators. 

There are ten sizes of cell, the smallest containing 
three plates 3 by 3 inches, and the largest having seventy- 
five plates i$y 2 by 30^ inches, ranging in capacity from 
5 to 12,000 ampere-hours, and in weight from 5^ to 




Fig. 124. Lead-Zinc Storage Battery. 

5,800 lbs. The smaller sizes are provided with either 
rubber or glass jars, and the larger one with lead-lined 
tanks. 

In the lead-lead cells the negative plates deteriorate 
in capacity, while the positive plates increase in capacity, 
with continued use. 

To even things up the two end plates are made nega- 
tive and they then alternate, thus giving one more nega- 
tive plate per cell. 



248 ELEMENTARY ELECTRICITY 

A. lead-zinc cell is made by the United States Battery 
Co. It is shown in Fig. 124. 

The positive plate is of perforated lead sheets riveted 
together with lead rivets and formed by the slow proc- 
ess of charging and reversal as described in first par* 
of lesson. The negative element is a zinc amalgam whicl 
swells up when charged. 

This amalgam lies on bottom of jar while the lead 
element hangs over it. 

The pressure given by these cells is a little higher 
than a lead-lead cell and they weigh less for the same 
capacity. For signal work they are excellent, while for 
reserve power use the lead-lead cell is preferred as being 
better under such severe conditions. 

The Edison Cell uses grids of nickel plated iron, the 
grids being filled with small nickel plated steel boxes 
which are perforated with very small holes. 

The boxes in positive plate are filled -with oxide of 
nickel and pulverized carbon, the negative boxes being 
filled with oxide of iron and pulverized carbon. 

The carbon in each case is merely to render material 
a better conductor. 

A 20% solution of caustic potash is used in a nickel 
plated steel vessel. 

The advantage of this cell is its lightness and ability 
to stand the most reckless abuse. For railway work it is 
no better than any other cell and its price puts it out of 
consideration. 






STORAGE BATTERIES 



249 






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Fig. 125. Top and Inside 
View of a Concrete Battery Well. 



Fig. 126. Battery Chute. 



250 ELEMENTARY ELECTRICITY 



Catechism. 

Question i. What is a storage battery? 

Anszver. It is a cell somewhat resembling a primary 
cell, used to store electricity. 

Question 2. Does the cell produce electricity or sim- 
ply store it? 

Answer. The chemicals in the cell are not normally 
capable of producing electricity but by treating them 
with electric current for some hours they become changed 
so that they can produce electricity like a primary cell. 

In fact we might say that a storage battery is a pri- 
mary cell which, when exhausted, is restored to its 
original power by application of electric current which 
renews the chemicals, whereas in the ordinary primary 
cell we have to buy new chemicals. The gravity cell 
can be renewed a few times by passage of current, but 
soon gets in a condition where purchase of fresh chemi- 
cals is absolutely necessary. 

Question 3. Of what maiterial is the positive plate 
made? 

Anszver. Red lead is the real plate, but it is supported 
by a lead grid. 

Question 4. What material is used for the negative 
plate ? 

Answer. Spongy lead or litharge, held in a lead grid. 
Zinc in form of amalgam lying on a copper plate. 

Question 5. What fluid is used in jars? 

Anszver. Dilute sulphuric acid. 

Question 6. Wnat is sulphuric acid? 

Answer, It is an oily liquid either colorless or with 



STORAGE BATTERIES 251 

a very faint yellow tinge. It is sold in carboys* as 
Oil of Vitriol and should test by hydrometer (Lesson 
15) to 1.842 specific gravity or 66 degrees Baume. 

To avoid the constant use of the decimal point battery 
attendants call the strong acid 1842 acid and call water 
1000 specific gravity. 

Question 7. \\ nat is dilute acid? 

Answer It is acid of 1842 strength mixed with pure 
water. To -make 1200 acid take 1 measure of acid and 
pour into 3 measures of water. For 1400 acid take 1 
measure acid and pour into an equal quantity of water. 
1200 acid is most used and corresponds to 25 Baume. 

Question 8. Is pure water necessary? 

Answer. For best results, yes. Distilled water is not 
so very expensive to make and it pays. Half of the 
battery troubles are caused by filling up cells with any 
clean water that is handy. 

Even clean water contains chemicals that should not 
get into the storage battery. 

Question 9. Should the diluted acid be cooled before 
putting in cells? 

Answer. It should be thoroughly cooled. Acid should 
always be diluted the day before you intend to use it. 
The specific gravity should be taken after acid has cooled 
at least twelve hours. 

Question 10. Does it make any difference whether 
1200 or 1400 acid is used? 

Answer. Yes. Acid from 1150 to 1230 is generally 
used. The stronger the acid the greater the capacity 
of the battery and the more liable it is to get the disease 
of Sulphate. 

*Carboys are large glass bottles several feet high and about the 
same diameter. They are securely boxed to prevent breaking. 



252 ELEMENTARY ELECTRICITY 

Question 1 1. Why is it that the acid tested in cells 
is sometimes so high? 

Answer. The acid is weakest when cell is discharged 
and strongest when cell is charged. This is because 
the acid goes in and out of the plates on discharge and 
charge. 

Question 12. What is meant by a 180 ampere-hour 
cell? 

Answer. It means that the cell in question will give 
1 ampere for 180 hours, if it has been fully and prop- 
erly charged. It might give 30 amperes for 6 hours if 
it was designed for allowing the flow of such a cur- 
rent. 

It certainly would not give 180 amperes for 1 hour 
as the heat generated would buckle the plates and ruin 
the battery before the end of the hour. 

Question 13. What is the normal rate of a battery? 

Answer. As batteries are usually made we may draw 
1 ampere from every 10 square inches of positive plate, 
counting both sides, without over heating. 

A cell of 5 positives and 4 negatives, has plates 5x7 
inches. What current is it safe to draw ? 7x5=35=0^ 
side of a plate. Both sides 70 sq. in. 5 plates gives 350 
sq. in. Dividing by 10 gives 35 as safe current. This 
is called the normal rate. In actual practice we usually 
discharge at about normal rate and hurry the charge by 
exceeding the normal rate. 

Question 14. What harm does this do? 

Answer. Wastes money. It costs more to put in 180 
A H (ampere hours) quickly than it does slowly. 

Question 15. What is an 8 hour rate? 

Answer. It means taking 8 hours to charge or dis- 
charge the battery. If battery is worked twice as hard it 



STORAGE BATTERIES 253 

will charge or discharge in half the time or at a 4 hour 
rate. 

Question 16. What precaution should be taken in a 
battery room? 

Answer. The room must be dry and well ventilated 
to get rid of the acid fumes. Walls and floors should 
be of enameled brick and ceiling of white cement. Win- 
dows should be white-washed on outside or of ground 
glass to prevent sun shining on cells and heating them. 

The benches cells stand on should be soaked in paraffin. 
The cells should stand on insulators. 

Question 17. How are signal batteries installed? 

Answer. In wells like Fig. 125 when there are many. 
These wells are of sheet steel and concrete, and are 
about 10 feet high. They are heated when necessary by 
small oil stoves or by being packed over the top with 
manure. 

When only two or three cells are used the battery 
chute of Fig. 126 is installed. The chute is of iron 
pipe and goes down below the frost line. The cells are 
hauled up by the rope for renewal or charging. 

Question 18. Of what material are battery jars made? 

Answer. Glass, hard rubber, celluloid, wood with 
sheet lead lining. 

Small cells usually have glass jars as in Fig. 127. 
Large cells have the lead lined wooden tank as in Fig. 
128. Hard rubber and celluloid are expensive. Neither 
is transparent. 

Question 19. How should cells be put in service? 

Answer. As soon as electrolyte (acid) is put in the 
cell it should be charged with one-third its normal 
rate for 4 hours, then increase to normal rate and con- 
tinue 20 hours. Cells will now be up to 2.6 volts each, 



254 



ELEMENTARY ELECTRICITY 



Drop back to one-quarter normal rate. The voltage of 
each cell will drop a little. Continue charging up to 
2.6 volts again. 




Fig. 127. Form of Storage Battery for Signal Work. Glass Jar. 



Question 20. How should cells in service be treated? 

Answer. Never discharge below 1.7 volts and 1.8 is 

better. Charge up to 2.5 volts usually at normal rate. 



STORAGE BATTERIES 255 

Once a week give a charge at one-third normal rate 
till cells read 2.6 volts. 

Never let them stand idle with less than 30% of their 
capacity in them. The fuller a cell the safer it is when 
idle. When cells are idle charge up to boiling once a 
week. 




Pig 128. Storage Battery in Lead Lined Wooden Tank. Tank Rests 
on an Insulated Frame. 



Do not habitually overcharge cells; it is a waste of 
money. 

The cell is charged when the specific gravity of elec- 
trolyte is about 0.025 higher than when discharged. 

Bubbles of gas are given off freely when cell is fully 
charged, because material of plate is no longer able to 
take up the oxygen and hydrogen which tend to be set 



256 ELEMENTARY ELECTRICITY 






free by the electrolysis;* these bubbles give the elec- 
trolyte the appearance of boiling, and often they are so 
fine that the liquid looks almost milky-white, particu- 
larly in a cell which has not been very long in use. 

The color of the positive plates varies from a light 
brown on active parts to a chocolate color when fully 
charged, and to nearly black when overcharged. The 
negatives vary from pale to dark slate color, but they 
always differ in color from the positives. This indica- 
tion of the amount of charge is learned by experience, 
but is quite definite after one becomes familiar with a 
particular battery. 

Do not discharge too rapidly, it wastes money. A cell 
whose normal rate of discharge is ioo amperes for 8 
hours, can be discharged at the rate of 400 amperes in 
one hour, but never at the rate of 400 amperes for 2 
hours. You see the rapid discharge is inefficient and 
you only get half as much energy out of cell as you 
could have obtained at a slower rate. 

Question 21. Do storage batteries wear out or de- 
preciate ? 

Answer. Yes, it will take 10% of the cost of a 
battery every year to keep it in repair.' 

Question 22. How should a battery be put out of 
commission or laid up? 

Answer. If, for any reason, the battery is to be but ! 
occasionally used, or the discharge is to be at a very 
low rate, a weekly freshening charge to full capacity at 
normal rate should be given. It sometimes happens that 
a storage battery is put out of commission for a long. 
period. In such cases the procedure is as follows: 
First the battery is given a complete charge at normal 

* Lesson 17. 






STORAGE BATTERIES 257 

rate, then the electrolyte is siphoned off into carefully 
cleaned carboys (as it may be used again), and as each 
cell is emptied it is immediately refilled with pure water. 
When the acid has been drawn from all cells and re- 
placed with water, the battery is discharged until the 
voltage falls to or below one volt per cell at normal cur- 
rent : when this point has been reached the water should 
be drawn off. In this condition the battery may stand 
without further attention until it is again put into service, 
which is accomplished in the same manner as when the 
battery was originally started. If during the discharge, 
when the water has replaced the electrolyte, the battery 
shows a tendency to get hot (ioo F.) colder water should 
be added. 

Question 23. What troubles occur in batteries and 
what are their remedies? 

Answer. The most serious troubles which occur in 
storage batteries are sulphating, buckling, disintegration, 
and short-circuiting of the plates. These can usually 
be avoided, or cured by proper treatment if they have 
not gone too far. 

SULPHATING. — The normal chemical reaction 
which takes place in storage batteries is supposed to 
produce lead sulphate on both plates when they are dis- 
charged, their color being usually light brown and gray, 
due to the presence of lead oxide,, on the positive plate. 
But under certain circumstances a whitish scale forms 
on the plates. Plates thus coated are said to be "sul- 
phated." This term is, however, somewhat ambiguous, 
the formation of a certain portion of ordinary lead 
sulphate being perfectly legitimate, but the word has 
acquired a special significance in this connection. A plate 
is inactive, and practically incapable of being charged, 



25S ELEMENTARY ELECTRICITY 






when covered with this white sulphate, as it is a non- 
conductor. 

The conditions under which this objectional sulphat- 
ing is likely to occur are as follows : 

(a) A storage battery may be left discharged for j 
some time, even though the limits have not been ex- j 
ceeded. 

(b) A storage battery may be overdischarged, that is, 
run below the limits of voltage specified, and left in that 
condition for several hours. 

(c) The electrolyte may be too strong. 

(d) The electrolyte may be too hot (above 125 F.). 

(e) A short circuit may cause "sulphating" because 
the cell becomes discharged (on open circuit) and dur- 
ing charging it receives only a low charge compared 
with the other cells of the series. A battery may be- 
come overdischarged or remain discharged a long time 
on account of leakage of current due to defective insu- 
lation of the cells or circuit, or the plates may become 
short-circuited by particles of the active or foreign sub- 
stances falling between them. 

(f) By charging at a very low rate, for example, 
one-thirtieth of normal. 

Sulphating may be removed by carefully scraping the 
plates. The faulty cells should then be charged at a low 
rate (about one-half normal) for a long period. In this 
way, by fully charging and only partially discharging 
the cells to about 1.9 volts at the 8-hour rate, for a 
number of times the unhealthy sulphate is gradually elim- 
inated. When the cells are only slightly sulphated, the 
latter treatment is sufficient without scraping; but with 
cells that are very badly sulphated, the charge should be 
at about one-quarter the normal rate for three days. 



STORAGE BATTERIES 259 

Adding to the electrolyte a small quantity of sodium 
sulphate, or carbonate,* which later is immediately con- 
verted into sodium sulphate, tends to hasten the cure of 
sulphated plates by decomposing or dissolving the white 
sulphate. This is not often used, as a cell should be 
emptied, thoroughly washed, and fresh electrolyte added 
before the cell can be used again. 

Sulphating not only reduces the capacity of lead stor- 
age batteries, but also uses up the active material by 
forming a scale which falls off or has to be removed. 
It also produces the following trouble: 

BUCKLING, or warping of a plate, may be caused by 
too great expansion of the active material, which strains 
the ribs of the containing grid : or by uneven action on 
the two surfaces ; for example, a patch of white sulphate 
on one side of a plate will prevent the action from tak- 
ing place there, so that the expansion and contraction 
of the active material on the other side, which occurs 
in normal working, will cause the plate to buckle. This 
might be so serious that it would be impossible to 
straighten the plate without breaking or cracking it; 
but, if taken in time, it may be accomplished by placing 
the warped plate between boards, and subjecting it to 
pressure in a screw or lever press. Striking the piare 
is objectionable, because it cracks or loosens the active 
material ; but, if it should be necessary to straighten a 
plate when no press is available, a wooden mallet may be 
used very carefullv, with flat boards laid under and over 
the plate. Buckling is caused by an excessive rate of 
charging or discharging, as well as by sulphating. 

DISINTEGRATION.— Some of the material may be- 
come loosened or entirely separated from the plates, as 

*Sal-soda; common washing soda. 



260 ELEMENTARY ELECTRICITY 

a result of various causes. The chief of these is sulphat- 
ing, which forms scales or blisters that are likely to 
fall off, thus gradually reducing the amount of active 
material and the capacity of the cell. Buckling also tends 
to disintegrate the plates. Contraction and expansion of 
the active material may take place in normal working, 
and are increased by excessive rates or limits of charg- 
ing and discharging. This constitutes another cause of 
disintegration, particularly in plates of the Faure type, 
containing plugs or pellets of lead parts. The fragments 
which fall from the plates not only involve a loss of 
active material, but are also likely to extend across or 
gather between the plates and cause a short circuit. 

The positive plates are far more susceptible to and 
injured by these troubles than the negatives. The former 
are also more expensive to make, therefore it is to them 
that special attention should be directed in the manage- 
ment of storage batteries. 

SHORT-CIRCUITING may be caused by conditions 
previously stated, and also by the collection of sediment 
at the bottom of the containing well. The short-cir- 
cuiting caused by the dropping in of foreign matter, or 
bridging by the active materials, is prevented by the 
use of glass, rubber, or wooden separators. The short- 
circuiting of plates by the formation of sediment is pre- 
vented, or the chances of it are decreased, by raising 
the plates so that they clear the bottom of the contain- 
ing cell. In small batteries this clearance is about an 
inch ; in large cells it is considerable, being about 6 inches, 
and on account of the weight of large-sized plates they 
are supported at the bottom by glass frames running 
lengthwise through the cell. 

The sediment should be watched carefully, and when 



STORAGE feATTERTES 261 

it reaches a depth of art inch or more at the center of the 
cells it should be removed-. The usual method is to tak£ 
out the plates, syphon the* electrolyte off carefully, and 
then flush out the tanks until all the sediment is re- 
hioved. If syphoning cdhnot be resorted to, a pump may 
be used, either of glass or of the bronze rotary type. 

TROUBLES FROM ACID SPRAY.— A battery will 
give off occasional bubbles of the gas at almost any time ; 
but when nearly charged the evolution becomes more rap- 
id. These bubbles, as they break at the surface, throw 
minute particles of acid into the air, forming a fine spray 
which floats about. This spray not only corrodes the 
metallic connections and fittings in the battery room, 
but is also very irritating to the throat and lungs, caus- 
ing an extremely disagreeable cough. Glass covers are 
sometimes placed over cells to prevent the escape of 
fumes, but this is not advisable as the glass becomes 
moist and will collect dust, thus forming a conducting 
surface over the battery. 

Attempts have been made to do away with the spray 
by having an oil film over the electrolyte, but this inter- 
feres with the use of hydrometers, and sticks to the sur- 
face of the plates when they are removed, thus increasing 
the resistance when they are replaced. Another plan 
consists in spreading a layer of finely granulated cork 
over the surface of the liquid, but while this does not 
interfere with the hydrometer, it makes the cell look 
dirty. The general practice is to depend almost entirely 
upon ventilation to get rid of the acid fumes, in fact, 
even forced ventilation is used. A blower forces fresh 
air into the room, which is provided with a free ex- 
haust. In connecting up the cells, it is advisable to use 
lead-covered copper cables, as this covering protects th$ 



262 ELEMENTARY ELECTRICITY 

copper, and prevents the formation of copper salts which 
might drop into the cell and contaminate the electro- 
lyte. 

THE PURITY OF THE ELECTROLYTE is very 
important, and great care should be taken to insure it. 
The electrolyte may have nitric acid present when 
"formed" (Plante) plates are used, and some chlorine, 
when "Chloride" negatives are used. In addition, iron 
may be present due to the water or acid, if the sulphuric 
acid is made from iron pyrites ; it may also be present, 
owing to the corrosion of iron fittings near the cells, 
some of the scale falling into the electrolyte. Similarly 
the copper salt formed from the connections by cor- 
rosive action may fall into the cell. Mercury may also 
be present due to the breakage of hydrometers or 
thermometers. Other foreign substance might be present, 
but those named are the most harmful. 

Nitric acid, even in exceedingly small quantities, causes 
disintegration, as the supporting metal grid of the plate 
is destroyed. 

Chlorine has a similar effect. 

Iron, mercury, and copper produce local action, and 
thus decrease the efficiency and ultimately the life of the 
cell. 

The electrolyte should be tested about once a week for 
these impurities, and if any of them are present, it should 
be drawn off and renewed. When nitric acid is found, 
it is advisable to flush the cell with pure water. 

Question 24. How are batteries connected to line? 

Answer. Usually they are "floated" on line, meaning 
that the battery is always connected and charged when 
load is light and discharged when load is heavy. This 
gives the battery the least possible work to do and keeps 



STORAGE BATTERIES 



263 



it well charged at all times. Fig. 129 shows this. 
Switches are provided to cut off battery or to cut off 
dynamo and let battery run the lights. Usually both 
switches are closed as shown. 

Question 25. What is end cell regulation? 

Answer. If 700 volts are wanted at station 300 cells 
would give 750 volts when fully charged and 510 volts 
when at their lowest safe limit. 




Pig. 129. Diagram of Connections for a Storage Battery to Float on Line. 



If 280 cells were connected permanently and 130 extra 
cells arranged so as to be cut in at the end of the string 
of 300 cells a few at a time, then when cells were down 
to 1.7 volts we would have 410 in service and have full 
voltage. 

Question 26. What is booster regulation? 

Answer. A booster is a dynamo whose field magnets 
are excited by the current going out to the lines. Hence 
when the battery is being worked the hardest the booster 
dynamo furnishes the highest voltage which is added to 
that of the battery. 



264 ELEMENTARY ELECTRICITV 

As the battery voltage drops, the attendant regulated 
the field rheostat of the booster so as to add enough 
voltage to keep the combined voltage up to the regular 
Voltage. 

Question 27. Are batteries much used in railroad 
Work? 

Answer. Yes. Every electrically lighted train has a 
storage battery to run things when the train is still; 

Many motor cars have storage batteries to operate th6 
controlling devices. 

Power houses have batteries to run things in case of 
accident. 

The New York Central has five batteries, each giving 
2,250 amperes for one hour, and others give 3,000, 3,750 
and 4,000 amperes each. The whole set can run the 
entire electrical division of the railroad for an hour in 
case of a mechanical breakdown. 



ELECTRO-MAGNETIC FIELDS. 

As has already been stated, a wire through which an 
electric current is flowing is surrounded by a magnetic 
field. Now, if the student views the wire from the end, 
these magnetic lines of force may be imagined as flowing 
in circular paths, as indicated in Figure 130. Assuming 
that the current is flowing away from the observer, the 
arrows in Figure 130 show the direction in which the 
magnetic stream is supposed to flow. 

There is no good reason for assuming that magnetism 
flows in the magnetic circuit, in the same manner as 
electricity flows in the electric circuit, but it is generally 
spoken of as if it did, owing to the fact that if a mag- 



ELECTRO- MAGNETIC FIELDS 



265 



netic needle be placed in the path of these lines of force, 
it will always turn its north end in the same direction 
with reference to the direction of the current of electricity 
flowing in the wire. Referring to Figure 130, suppose 
the current were flowing downward through the wire ; 
if a magnetic needle were placed where one of the arrows 
is drawn, the north end of the needle would turn in the 
direction in which the arrow points. If the wire carry- 
ing the current is wholly surrounded by air, the mag- 
netic lines of force will be circles as shown in Figure 130, 




Fig. 130. 



Fig. 131. 



with the wire as the center of these circles ; but if there 
is any iron or steel present, the form of these lines of 
force will be materially changed. For instance, if a 
piece of iron of the shape shown in B, Figure 131, was 
located relative to the wire A as indicated, the shape and 
position of the lines of force would be changed, for the 
simple reason that iron, being a much better conductor 
of magnetism than air is, nearly all of the lines of force 
would flow through it. This is shown in Figure 131 by 
the greater density of the lines of force between the ends 



266 ELEMENTARY ELECTRICITY 

A r — S" of the ring B than in the space within it. If the 
ring B were made solid, the greater part of the mag- 
netism, in fact, nearly all of it. would be confined to 
the ring: but if a piece be cut out of one side, as shown 
in Figure 131, a greater proportion of the lines of force 
will travel the interior space, and they will increase in 
number as the piece cut out is increased in size. Should 
the wire A through winch the electric current is passing 
be moved around within the ring B. the only effect will 
be to change the path of the lines of force in the interior 
space, but those located in the iron will remain practically 
unchanged. If electric currents are passed through a 
number of wires, the lines of force surrounding each 
wire will be in the direction corresponding to the cur- 
rent passing through that particular wire, and if these 
wires are near each other, the magnetism of one may 
assist that of the other, or it may act in opposition to it 
In Figure 132 a number of cases are represented. Con- 
sider first the two wires A A, in both of which the elec- 
tric currents are flowing in the same direction, as is in- 
dicated by the arrows A' A'. In the two wires B B die 
currents are supposed to be flowing in opposition direc- 
tions, and as a consequence the lines of force are oppo- 
sitely directed. The effect of these two cases can be 
realized by comparing the wires C C and E E. In these 
two diagrams the wires are drawn close enough together 
to show the effect of the lines of force upon one another. 
In the case C C, the two sets of lines can join and flow 
together around the two wires, forming one single path, 
but in the case E E the lines meet each other end on, 
and as a result they neutralize each other, hence, if in 
the two wires the electric currents flow in the same direc- 
tion, the effect is to increase the magnetic force, if they 



ELECTRO-MAGNETIC FIELDS 267 

are placed together, for the reason that the lines of force 
of the two wires are added together. On the other hand, 
if the electric currents in the two wires are flowing in 
opposite directions, the result of placing them close to- 
gether is a reduction of the magnetic force, because one 
neutralizes the other. In the first case, if the currents 
flowing in both wires are equal, when the wires are placed 
side by side, the magnetism surrounding the two will be 
twice as great as that surrounding each one when they 
are placed at some distance from each other. In the 



A 



1 / I J ^ \ 



fc'#e.^ ^ 



\ . . 

Fig. 132 



a'U*. \ i <m,^~S i - \ 



o 



M «H -- V 



IE) 
/ 

/ 



second case, if the currents in the two wires are of equal 
force, the effect of placing them side by side will be to 
completely destroy the magnetism, for the reason that 
the lines of force of one wire will be just sufficient to 
counterbalance and head off the lines of the other wire. 
In the case of the four wires D D D D, if the electric 
currents in all are in the same direction, their magnetisms 
will help one another, and if they are placed side by side, 
the lines of force will join and pass around all the wires, 
as shown in the diagram. In these diagrams the lines 



268 



ELEMENTARY ELECTRICITY 



of force are shown as curving in and out, between the 
wires, but as a matter of fact they would pass along in 
straight lines from wire to wire ; the curved form has 
been shown so as to illustrate more clearly how the long 
lines surrounding all the wires are built up of the sev- 
eral small circles surrounding the individual wires. 




Fig. 133. 



If the current flowing in each one of the wires 
D D D D is equal in strength to the current flowing 
in the wires A A, then it is self-evident that the number 
cf lines of force — that is, the strength of the magnetism 
around each of the D wires — will be the same as that 
around each one of the A wires, and if such is the case, 
it is equally evident that, as the lines that flow in the 
long path around all of the D wires are the sum of all 



ELECTRO-MAGNETIC FIELDS 269 

of the lines in the path around each wire, the lines of 
force surrounding the four wires are four times as many 
as are those around each individual wire. From this it 
day be clearly seen that, by placing side by side a large 
number of wires, and passing through all of them an 
electric current flowing in the same direction in each 
wire, a strong stream of magnetic lines of force may be 
developed. Referring to Figure 133, it is evident that 
all that is required to accomplish this result is to wind a 
wire in the form of a coil, for the reason that the cur- 
rent will then pass through all of its turns, flowing in 
the same direction. This is clearly shown by the arrows 
A and the lines B and D, which latter indicate the path 
of the lines of force on two sides of the coil. It will be 
understood that the lines of force will surround all sides 
of the coil. Should it be desired to increase the number 
of lines of force surrounding the coil C, it may be ac- 
complished by either increasing the strength of the elec- 
tric current flowing in the wires or by increasing the 
number of turns of wire in the coil. 

As was shown in connection with Figure 131, a mass 
of iron placed around a wire will divert the lines of force 
to itself, owing to the fact that it is a much better con- 
ductor of magnetism than air. Keeping this in mind, it 
is a simple step from the plain coil in Figure 133 to the 
U-shaped magnet of Figure 134, for this is simply a 
case in which the lines of force of a simple coil, such 
as is shown in Figure 133, are slightly diverted from the 
path they would take if there were no iron present. 

At this point it may be well to add that the presence 
of the iron not only diverts the lines of force from the 
natural path, but also increases their number, for as 
the path through the iron offers much less resistance, the. 



270 



ELEMENTARY ELECTRICITY 



same amount of electric current flowing in the wires will 
develop a greater magnetic strength. 

The type of magnet shown in Figure 134 is used ex- 
tensively in electric machines, especially in small motors. 
Another form of magnet which can be developed from 




Fig. 134. 



this is that in which there are two coils, one on each 
side of the armature. To construct such a magnet from 
Figure 134 it would be necessary to make two magnets 
similar to that in the figure, and then bring them together 
with the two A* poles on one side and the 5 poles on the 
other. If the coil of Figure 133 is increased in length 



ELECTRO-MAGNETIC FIELDS 



271 



and then drawn apart in the middle, a form will -be ob- 
tained similar to that shown in Figure 135, and the lines 
of force will pass through the two halves of the coil, 
as indicated by the broken lines. If the coil be now bent 
so that the two halves are parallel with each other, as 
shown in Figure 136, then the lines of force on one side 
will still continue to flow through both parts of the coil, 
as indicated by the broken line A ; but the other lines will 




Fig. 135. 



Fig. 136. 



Fig. 136a. 



necessarily break in the middle and form two independ- 
ent paths, as shown at B and C. Their direction through 
the halves of the coil, however, will be the same as be- 
fore, and the only reason why they break is that by so 
doing they avoid traversing a considerable amount of 
unnecessary space. 



272 ELEMENTARY ELECTRICITY 

The two coils of Figure 136, which, as we have shown, 
are simply parts of one and the same coil, can readily be 
applied to a magnet of the form shown in Figure 136", 
and thus we arrive at that class of magnets in which two 
coils are used, and we find that they in no way differ in 
principle from the more simple variety in which only one 
coil is required. A comparison of the magnets in Fig- 
ures 134 and 136" will show that there is no difference 
between them except in the location of the coils. In both 
it will be noticed that the lines of force have one path 
from end to end of the magnet, as shown by line A, in 
both figures, and one path B in Figure 134, and two 
paths B and C in Figure 136% which are not from end 
to end of the magnet. In electrical machines the main 
path from one end to the other is the only part of the 
magnetism that is made useful. The lines passing along 
the other paths are of no service, and are generally called 
the stray field. 

In Figures 133 and 136 the lines of force passing 
along the several paths are practically equal, but in Fig- 
ures 134 and 136° such is not the case. As has been 
already stated, iron is a much better conductor of mag- 
netism than air is, therefore as the proportion of the line 
B in Figure 134, that passes through the air, is greater 
than that of line A, the magnetism along the latter path 
will be the greater. 

In properly constructed electrical machines the space 
between the ends of the magnet is nearly all filled up 
with the iron core of the armature, and this reduces the 
length of the air portion of the lines of force passing 
along the main path to such an extent that the stray 
field through the other paths is usually but a small pro- 
portion of the total number of lines of force developed 



Electro-magnetic fields 



m 



by the coils. The object aimed at by designers of elec- 
trical machines is to reduce as much as possible the 
length of the air portion of the main path of the mag- 
netism, and to make that of all other paths as great as 
possible, so as to reduce the stray field to the lowest 
point. 

TABLE OF PERMEABILITY 



WROUGHT IRON 




CAST 


IRON 




o 

<*£ 

<x> •— ' 

ftO> 

§3 

h4CQ 


Permeability 
or Multiply- 
ing Power 
of Iron. 


< 

a 

o> 
p 
3 


Ampere 
Turns per 
Inch in 
Length 


o 

ftOJ 

.5 <y 

►JOG 


Permeability 
or Multiply- 
ing Power 
of Iron 


< 
a 

02 
V 

a 
3 


Ampere 
Turns per 
Inch in 
Length 


30,000 


3,060 


9.8 


3.06 


25,000 


833 


30.0 


9.4 


40,000 


2,780 


14.4 


4.72 


30,000 


580 


51.7 


10.2 


50,000 


2,488 


20.1 


6.29 


35,000 


390 


89.7 


27.5 


60,000 


2,175 


28.0 


8.76 


40,000 


245 


163. 


51. 


65,000 


1,980 


32.8 


10.26 


45,000 


135 


333. 


104. 


70,000 


1,920 


40.7 


12.7 


50,000 


110 


454. 


142. 


75,000 


1,500 


50.0 


15.6 


60,000 


66 


909. 


284. 


80,000 


1,260 


63.5 


19.8 


70,000 


40 


1750. 


548. 


85,000 


1,030 


82.5 


25.8 










90,000 


830 


108.0 


33.8 










95,000 


610 


156. 


48.8 










100,000 


420 


238. 


74.5 










105,000 


280 


375. 


117. 










110,000 


175 


629. 


197. 










115,000 


95 


1210. 


378. 










120,000 


60 


2000. 


626. 










125,000 


40 


3125. 


978. 










130,000 


30 


4333. 


1356. 










135,000 


24 


5626. 


1761. 










140,000 


18 


7777. 


2434. 











274 ELEMENTARY ELECTRICITY 



MAGNETIC TRACTION. 

It is a well-known fact that the north and south poles 
of two magnets attract each other. In fact, the magnetic 
lines act as if they were elastic cords that always tend to 
shorten themselves. When a great number of the mag- 
netic lines flow across a given space the attraction is very 
strong. 

Ewing, in some of his experiments, pushed the magne- 
tism of a piece of soft iron to such a point that the pres- 
sure due to the magnetic attraction was 1,000 pounds per 
square inch. 

The following table gives the pull between a magnet 
and its armature when the given fluxes pass. The for- 
mula from which this table was calculated is 






Pull in pounds equals (7) 

r ^ 72,134,000 w ' 

in which B equals the flux in lines per square inch, A 
equals the area of cross section between magnet and arma- 
ture in square inches. 



ELECTRO- MAGNETIC "FIELDS 275 



Flux per sq. in. between Pull in lbs. per sq. in. between 

armature and magnet. armature and magnet. 

5,000 .34 

10,000 1.4 

15,000 3.1 

20,000 5.5 

25,000 8.7 

30,000 12.5 

35,000 20.0 

40,000 22.2 

45,000 28.1 

50,000 34.6 

55,000 41.9 

60,000 49.9 

65,000 58.5 

70,000 67.9 

75,000 78.0 

80,000 - 88.7 

85,000 100. 

90,000 112. 

95,000 125. 

100,000 138. 

105,000 153. 

110,000 168. 

115,000 183. 

120,000 199. 

125,000 216. 

130,000 234. 

135,000 . 252. 

140,000 272. 



LESSON if. 
Electrolysis; 

The word electrolysis means a loosening up by elec- 
tricity done in a liquid; 

There are three classes of liquids I 

(i) Do not conduct electricity, as oils, petroleum 
products. 

(2) Simply conduct like mercury, melted metals. 

(3) Conduct and are loosened up so that the different 
constituents separate and the constituents of the same 
kind collect together. 

Dilute acids ( 1 part acid and 20 parts water) ; solutions 
of metallic salts (copper sulphate, ammonium chloride) ; 
melted chemicals ; are in the third class. 

Liquids of the third class are called Electrolytes and 
the process Electrolysis. 

Now when an electric current is passed through these 
solutions, they split up into parts, one part being liberated 
at the point where the current enters, and the other part 
where it leaves the liquid. If, for instance, we pass a 
current through water, we find oxygen gas being liber- 
ated where the current enters the water, and hydrogen gas 
where it leaves. The conductors that lead the current 
into and out of the liquid have been called the electrodes 
(or electricity doors). The leading- in electrode is called 
the anode (or entering door), and the leading-out one the 
cathode (or exit) . Therefore we say oxygen is liberated 

276 



ELECTROLYSIS 



2?7 



at the anode, and hydrogen at the cathode. If the solu- 
tion contains a metal it is always liberated at the cathode. 

The plus wire of circuit is attached to anode and nega- 
tive wire to the cathode. 

When a metal is dissolved in a dilute acid and the water 
boiled away the solid substance left is called a salt. 

If the metal sodium is dissolved in weak muriatic acid 
and the water boiled away common table salt is left 
behind. If this is dissolved in water again we can by 
electrolysis separate it into the soda and muriatic acid 
again. 

Cryolite is a compound with a great deal of aluminum 
in it. By melting it and while liquid passing current 
through it the aluminum is collected at the cathode. 

The pieces of the electrolyte produced by electrolysis 
are called ions. 

HOW ELECTROLYSIS TAKES PLACE. 

Electricity is an invisible something known only by its 
effects. It can be moved from place to place through 
the air as Marconi has shown, or it can be more accurately 
and more cheaply transferred by copper wires. How the 
air or the copper wire conducts the electricity we do not 
know. 

When electricity is transferred through a liquid we 
know that certain kinds of little particles of the sub- 
stances in the liquid carry the electricity across from the 
anode to the cathode and stay there and certain other 
kinds of particles collect about the anode, for there is a 
transfer of electricity in both directions. 

Unless these little particles are present the liquid will 
not conduct. If they are present the liquid conducts and 



278 ELEMENTARY ELECTRICITY 

while conducting the loosening process goes on and more 
particles (ions) are produced to keep up the conductivity 
of the liquid. 

ELECTROLYSIS OF WATER. . 

Take a glass of boiled and filtered water (it would be 
better if it were distilled water). Bring the + and — 
wires from a 3-cell battery to the glass and fasten strips 
of platinum foil to the ends by wrapping on wires. Bend 
the wires up and over the edge of glass and let platinum 
strips hang in water. Do not let the copper wires get 
even wet, much less in the water. An ammeter would 
show no current passing -because the pure water has no 
ions in it. Now pour in a teaspoonful of sulphuric acid 
and the water begins to conduct (due to presence of ions) 
and electrolysis commences. 

The water is composed of hydrogen and oxygen in the 
proportion of 2 to 1, and the electrolysis allows these 
gases to escape into the air so that after a while all the 
water will be turned to gas. 

To a railroad man electro plating and destruction of 
water and gas pipes are the two important things. 



ELECTRO PLATING. 

Electro-plating and chemical-plating are often mixed 
up in people's minds. 

If you thrust a pen knife blade or a key into the copper 
sulphate solution used in a gravity cell, the knife is in- 
stantly copper plated by chemical action. If a sheet of 
iron, sprinkled with sal ammoniac is dipped in a bath of 
melted zinc it is chemically plated and is called galvanized 



Electro platIng 



279 



iron. Electricity had nothing to do with either of these 
platings. 

As we know that the metals are dissolved into the 
solution at the anode and deposited at the cathode, we 
may electroplate an article with copper in the following 
way: 




Fig. 137. Electro-plating Cell. 



Make a bath of i gallon water, 10 oz. potassium cyanide 
(deadly poison), 5 oz. of copper carbonate, and 2 oz. 
potassium carbonate. 

Place in a glass or wooden tub, connect to positive 
wire a slab of copper (Fig. 137), and to negative wire 
the thoroughly cleaned articles. The passage of a cur- 
rent of low voltage will plate copper on the articles. 



280 ELEMENTARY ELECTRICITY^ 

Brass may be electro-plated on iron castings. This is 
a cheap and nasty substitute for a solid brass casting. 

Impure ores of copper, scrap copper, old telegraph and 
electric light wires in which there is always more or less 
solder, and copper mixed with other impurities, may be 
purified by electrolysis, the cathode being a plate of pure 
copper, the bath a solution of sulphate of copper, and 
the anode the impure metal. The pure copper replaces 
the exhaust from the bath, and the impurities fall to the 
bottom of the tank. Recovered copper thus deposited is 
extremely free from impurities, and is used for electrical 
conductors where low resistance is required. 



ELECTROLYTIC CORROSION. 

When the current has passed through motors it returns 
to the power house by many paths, some of which are: 
Rails of the track, return feeders between rails, elevated 
railway structures, waterpipes, gaspipes, cables of tele- 
phone wires, Edison tubes, adjacent streams of water. 

Let us consider the water and gaspipes and the cables. 
The current going into these is positive and when finally 
the current leaves them to go to the earth plates of the 
power house, if the ground is the least bit moist, metal 
is dissolved and taken away. In this way holes have been 
eaten in gas and water mains, and the sheathing of cables 
destroyed. The return current has a habit of leaving the 
pipes at each joint and coming back into pipe on the 
other side. This causes corrosion at every joint. 

This electrolytic corrosion is frequently the cause of 
law suits against the railroad companies. 

To reduce the evil and to be able to show in case of 



ELECTROLYTIC CORROSION 281 

suit that every possible precaution has been taken the 
company should: 

Thoroughly bond the track rails at every joint. 

Run a bare copper wire along between the rails and 
connect each rail of the track to it. 

Whenever a pipe or cable is found to be in danger of 
corrosion, run a wire from negative pole of station to 
the pipe and make a well soldered connection. 

It is evident that alternating current will not cause 
corrosion for it is rapidly reversing in direction. 

Main line tracks will have far less trouble with corro- 
sion than branches, and city extensions, belt lines, etc., 
which run through streets crowded with pipes and cables. 



::: 



It 



CIRCUITS. 

An electric current is so called because it is the thing 
through which the electricity passes or makes its circuit 
around through the different pieces of apparatus and 
machinery. 

The word circuit is used in connection with many 
other words as : series circuit, parallel circuit, short cir- 
cuit A. C. circuit, etc. These will all be explained in the 
following lessons. 

A dead circuit must be made live before it can deliver 
power, and where so delivering it is called a loaded cir- 
cuit. 

Every loaded circuit has Conductance. Resistance, In- 
sulation, Pressure and Current which are explained in 
the following lessons. 

The habit of referring to circuits as lines has grown 
so that the words are almost interchangeable. 

We ought to be more accurate and use the words in 
this manner. 

When a wire starts from the power house and returns 
again we have a circuit When we speak of a part of 
this circuit we say "the line between Chicago and Hins- 
dale.'" 

When the wires run from a power house to a sub-sta- 
tion we may call it ''the line." Of course there is an 
electric circuit there but we are always thinking of it as 
if current only flowed from power house to sub-station, 

and speak accordingflv. 

2S2 



CIRCUITS 



283 



W 



hen one side of the circuit is composed of rails, 
earth, etc., we always speak of the copper part as the 
"line" and the rest of it as the "ground." 

Fig. 138 shows several kinds of circuits and corre- 
sponding effects. 

Taking the top part of the figure with the solid lines. 
Starting from the terminal of the battery we have a 
series circuit through the magnet, lamp, resistance, elec- 
trolytic tank and back to the terminal of the battery. 




U__~] 



Fig. 138. Diagram of Circuits. 



In sueh a series circuit the same current must pass 
through each of the pieces of apparatus. If the circuit 
had all lamps, or all electro-plating tanks this would per- 
haps be satisfactory, but since a lamp takes one-half to 
one ampere and the plating bath 10 amperes and up- 
wards, it is evident that such a circuit will not work at 
all when different kinds of apparatus are in it. Either 
the plating bath won't work or the lamp will be burnt out 
in a few minutes. 



284 ELEMENTARY ELECTRICITY 

Another objection to a series circuit is that should we 
wish to stop the flow of current through the resistance 
R, we cannot open the circuit by a switch as that would 
cut the current ofr* from all the rest of the apparatus. A 
short-circuiting switch must be installed as shown at a, b. 
When the ends of these wires are connected by a switch, 
a shunt circuit is formed around R and the shunt will 
carry the current when R is removed. The shunt cir- 
cuit must be made of large enough wire to carry the cur- 
rent formerly carried by R without over heating. 

Such a short circuit of a part of a series circuit causes 
no trouble at all but a short circuit of the whole of a 
series circuit must be made more carefully. The re- 
sistance of the wires x, and y forming the s'hort circuit 
should be very low when a series dynamo is used and 
verv high when a battery is used. 

A very low resistance short circuit on a series dynamo 
stops the generation of current, and a very high re- 
sistance short circuit on a battery stops it generating 
current. 

A high resistance short circuit is always referred to 
as a shunt, and the words short circuit reserved for 
low resistance ones. 

Looking at the dotted part in Fig. 138 we have two, 
mains running from the battery and branches running 
across between mains ; the branches containing the ap- 
paratus. 

The mains and branches form parallel circuits and 
the apparatus is said to be installed in parallel or mul- 
tiple. 

Several distinct advantages are gained by the par* 
allel system, 



circuits 285 

Each piece of apparatus can draw as much current 
as it needs without interfering with other apparatus. 

The current may be cut off from any of them by 
opening a switch in its branch without affecting the 
remaining branches. 

Short circuits (i. e. low resistance) anywhere in a 
parallel circuit will cause considerable damage. 

Parallel circuits are fed by shunt dynamos or alter- 
nators. A short circuit on a shunt dynamo causes it 
to generate an enormous current and may destroy the 
apparatus short circuited and the dynamo's armature. 
Alternators do not produce such large currents when 
short circuited so in this case it is the apparatus short 
circuited that suffers most. 

When each branch of a parallel circuit contains sev- 
eral pieces of apparatus in series, the whole is called a 
series parallel circuit. 

Suppose we now learn how electricity flows through 
circuits. 

In hydraulic, pneumatic or steam engineering, the in- 
dications of the pressure gauge are of the utmost im- 
portance to the engineer; in fact, he is always consid- 
ering and asking about the pressure, and does not trouble 
himself about the water, air, or steam, for none of these 
would be of any use to him unless they existed under 
a certain head or pressure. It is simply the pressure 
under which they exist that gives to them their working 
power. 

In a similar way the electrical engineer is always 
concerned about the electrical pressure; he does not 
talk or think much about the electricity, but the elec- 
trical pressure is always in his mind as being of th$ 
first importance. 



286 ELEMENTARY ELECTRICITY 

The hydraulic engineer measures his pressure in 
pounds per square inch, that is to say, his unit of pres- 
sure is that exerted by a pound weight. The electri- 
cal engineer's unit of pressure is called the volt (from 
Volta, an Italian electrician), the consideration of which 
we will leave for a future lesson. It is owing to this 
pressure that a current of electricity flows around a con- 
ducting circuit. No current could possibly flow unless 
there was a difference of electrical pressure in the cir- 
cuit, in the same way that no water would possibly flow 
through a water conductor unless there existed a dif- 
ference of pressure. 

Instead of calling it the electrical pressure, we might 
call it the electricity-moving force, or the electro-motive 
force, or, for brevity, the E. M. F., which is the term 
most commonly applied to the electrical pressure; thus 
we speak of the E. M. F. of a circuit as being equal to 
so many volts. 

It would perhaps be well to point out here the en- 
gineer's meaning of pressure.* 

We may exert a pressure and still have no resultant 
motion ; as, for example, suppose a man applies a pres- 
sure (a moving force) at one end of a table, which 
would of itself be able to move the table along the 
floor ; if now a boy pushes at the opposite end and in the 
opposite direction, it is certain that the table would not 
be moved as rapidly as before, providing the man pushes 
with the same force throughout, while if another man 
takes the place of the boy and pushes with equal force 
to the man opposite to him, the table would not be moved 
at all, although there is now a greater pressure being 



*This illustration is taken from Mr. Tyson Sewall. 



ELECTRO- MOTIVE FORCE 287 

applied to the table than in the original case. It will 
therefore be seen that the result obtained does not de- 
pend on the pressure, but on the difference of pressure, 
and in all cases where pressure is spoken of it is the 
difference of pressure that is meant. 

Returning to the experiment with the table, we have 
just seen that before the table can be moved we must 
provide a table-moving force, but when this is provided 
it does not follow that the table will move even then — 
it all depends on the resistance offered. We can imag- 
ine a very heavy table, with rough feet, standing on 
two rough boards, and the man applying a moving force 
to it but producing no movement; whereas when the 
floor has been smoothly planed he may be able to move 
it slowly, and by fitting wheels or casters to the feet of 
the table he may be able to move it rapidly with the 
same moving force. 

We see that the rate of movement of the table depends 
partly upon the table-moving force applied and also in 
part upon the resistance offered by the boards on which 
the table stands. It is directly proportional to the 
former and inversely proportional to the latter. That is 
to say, if we double the moving force while the resistance 
remains the same, the rate of movement of the table will 
be doubled, and if we keep the moving force the same 
and halve the resistance, the rate of movement of the 
table will be doubled. This could be stated thus : 

Rate of move- ) table-moving for ce 

ment of table [ is Proportional to res i st ance. 

Therefore the rate of movement of the table is a 
thing entirely dependent on two other things, and in try- 



288 ELEMENTARY ELECTRICITY 

ing to find its value we have to ask, first, what is the 
moving force available? and second, what is the resist- 
ance offered? The same applies to water moving in 
pipes and this is perhaps a better analogy to the elec- 
trical case. We say there is a current of water flowing 
through the pipes, but this current is flowing simply be- 
cause there is a difference of pressure between the two 
ends of the pipes, and as the pipes offer a certain resist- 
ance, while these two things remain constant, the 
strength of the current will remain constant also. If we 
desire to alter the rate of flow (the current), we must 
alter either the water-moving force (the head) or the 
resistance (the tap). We can now see why the engineer 
is not concerned so much about the current; he may 
want a certain current to flow, but he gets it by seeing 
either to the water-moving force, or to the resistances^ 
or to both. 

If we now apply this electricity we find the same ideas 
in the mind of the electrical engineer. If he desires a 
certain current he asks himself, "What E. M. F. (elec- 
tro-motive force) have I available ?" and then, "What re- 
sistance must I have in the circuit ?" and he makes all 
alterations in the current strength by adjusting the one 
or the other, or both, to suit. If the circuit has a fixed 
resistance then he cannot alter the current flowing round 
it except by proportionally altering the E. M. F., that is 
to say, if he wishes to have twice the current strength he 
must put twice the E. M. F. into. the circuit. If the E. 
M. F. has a fixed value, then he cannot alter the current 
without altering the resistance of the circuit, thus — if he 
wishes to double the current strength he must halve the, 
total resistance of the circuit. 



FLOW OF CURRENT 



289 



All the so-called generators of electricity, dynamos, 
batteries, etc., are simply devices for producing and 
maintaining an E. M. F. Some dynamos produce high 
E. M. F.'s from 2000 to 10000 volts and even higher, 
while battery cells produce low E. M. F.'s from 1 to 2 
volts only. 

To make the analogy between the water system and 
the electrical system more correct, we should suppose a 
closed circuit of pipes, as shown in Fig. 139, completely 
filled with water, having a rotary pump P in the cir- 
cuit, and furnished with a tap R a pressure gauge PG, 
and a current gauge C G. 




Fig. 139. Simple Hydraulic Circuit with Gauges. 



Suppose now we turn on tap T and start the pump 
working, the pressure gauge will indicate a difference of 
pressure in the circuit, and the current gauge will indi- 
cate a current, flowing round the circuit. 

In this case we are not generating water, we are sim- 
ply putting into motion the water that was already there, 
and this is done by creating a difference of pressure by 
means of the pump, and by providing a conducting cir- 
cuit. If we stop the pump, the two indicators will point 
again to zero, but there is just the same amount of water 
there as we started with, there has been no consumption 
of water. 



290 ELEMENTARY ELECTRICITY 

In the same way we have to think of an electrical cir- 
cuit (Fig. 140). The dynamo D is simply a device for 
producing a difference of electrical pressure, and is put 
into the circuit to act exactly as the pump does in Fig. 

139 if we switch on S, which is comparable with turning 
en the tap in Fig. 139, and start the dynamo working, 
the pressure gauge (called the voltmeter) VM will indi- 
cate a difference of electrical pressure or an E. M. F., 
and the current gauge (called an ampere-meter, or for 
brevity an ammeter) will indicate a current flowing 
round the circuit. In this case we are not generating 
electricity, but simply putting into motion electricity that 
was already there. 

Going back to Fig. 139., suppose we turn off the tap T 
and keep the pump working, then the current meter will 
indicate no current, but the pressure gauge will indicate 
a slightly higher pressure than before. Here we have a 
water-moving force, but the resistance in the circuit is 
now exceedingly great, consequently no current can flow. 

Similarly in Fig. 140 if we switch off, still keeping the 
dynamo running, the voltmeter will show a slightly 
higher pressure, while the ammeter will indicate no cur- 
rent. Here again we have the one essential for a flow 
of electricity round the circuit, but not the other, for in 
switching off we have introduced into the circuit an 
enormous resistance. 

It will be noticed that we have referred throughout to 
the current as being not the movement of the table, or 
the flow of water or electricity, nor yet the quantity* of 
water or electricity moved, but as the rate of movement. 
Fifty gallons of water is not a current of water, but 50 
gallons per minute is a statement of the rate of flow, and 






FLOW OF CURRENT 



291 



consequently is a statement of the current strength. A 
current is the rate of flow. 

Let us return again to our water circuit (Fig. 139) 
and examine it more closely. Imagine the system to be 
quite full of water, and remember that water is a practi- 
cally incompressible fluid. If now we have our pump at 
work with the tap turned off, we shall have a difference 
of pressure between the two sides of the pump but no 
current. The moment we turn the tap on the current 




Fig. 140. A Simple Series Circuit, with Instruments. 



will flow, but this current will be everywhere in the cir- 
cuit of the same strength, it will not be strongest at the 
pump and get weaker as we go round the circuit, but 
will instantly have the same strength everywhere, and 
the current does not get used up in going round the cir- 
cuit. 

This is exactly the case with the current in Fig. 140. 
J* the dynamo be at work we have an E. M. F. in the 
circuit, but while the switch is off no current can flow. 
The moment we switch on there is a current in the cir- 



292 ELEMENTARY ELECTRICITY 

cuit which is everywhere of the same strength, not 
stronger near the dynamo, and getting used up as it goes 
round the circuit, but of the same strength everywhere 
in the circuit. 

It is evident that flow of current is regulated by pres- 
sure and resistance of circuit so that 

_ Resistance 

Current^ = 

Pressure 

using the principles taught in Formulas Page 210 we 
get two other forms of this rule 
Pressure = Current X Resistance 

Pressure 






Resistance = 



Current 



A man named Ohm first noticed this rule and it is 
called Ohm's Law in his honor. 

Let us now consider a few problems on the Ohm's 
Law. 

1. The E. M. F. in a simple circuit (Fig. 140) is 100 
volts, the resistance of the whole circuit is 50 Ohms. 
Wrtat current will flow through the circuit? 

n , , . ' ~K M. F. 

Ohm s law says, current = 



Resistance 



Therefore in this case C= = 2 amperes. 

It must be fully realized by the student that while the 
E. M. F. remains at 100 volts, and the resistance re- 
mains at 50 ohms, the current in that circuit will be 2 
amperes, no more and no less. It is impossible for any 
other strength current to flow. 



OHM'S LAW 293 

2. The resistance of the circuit being reduced to 10 
ohms, while the E. M. F. is kept at leo volts, what is 
now the strength of the current? 

E. M. F. 
Again current 



Resistance 



Therefore C = — =10 amperes. 

3. It is found that when ah E. M. F. of 100 volts is 
applied to a circuit, a current of 25 amperes flows. What 
is the total resistance of the circuit? 

b . . E. M. p. 

Resistance^ 

current 

*f-, r •-, 100 volts , 

Therefore resistance == ass 4 ohms^ 

25 amperes 

4. In the same circuit we find that by twisting upon 
itself some of the wire of which it is composed, the cur- 
rent increased to 50 amperes. What is now the resistance 
of the circuit, and how much resistance has been cut out 
by so twisting up the wire? 

Again by Ohm's law — 

D . - E. M. F. 

Resistance = — 

current 

™« , . 100 

Therefore resistance = — =2 ohms. 

It had four ohms previously ; we have therefore cut out 
4 — 2 = 2 ohms. 



294 ELEMENTARY ELECTRICITY 

5. In a circuit of 20 ohms resistance, a current of 5 
amperes is flowing. What is the E. M. F. in the circuit? 

By Ohm's law — 
The E. M. F. = Current^ Resistance. 
Therefore E. M. F.=5X20=ioo volts. 

6. What is the E. M. F. in a circuit whose resistance 
is equal to 10 ohms when a current of 20 amperes flows 
through it ? 

E. M. F.=^CurrentX Resistance. 
E. M. F,.=-20 X 10 = 200 volts. 

We have now to consider circuits other than the simple 
circuits just described, known as divided circuits or par- 
allel circuits. 

Fig. 141 represents a simple circuit in which the prin- 
cipal resistance consists of a conductor A of 50 ohms re- 
sistance; the remainder of the circuit consists of a dyna- 
mo capable of generating, the E. M. F. of 100 volts, and 
thick connecting wires joining it to the ends of A. 

If we neglect for the present the smaH resistances of 
the dynamo and connecting wires, then the current flow- 
ing is 

■■ E 100 
C= ?p~= = 2 amperes. 

Suppose we now join the points C and D with another 
conductor B exactly similar to A, as in Fig. 142. Will 
the current through the dynamo be greater or less than 
before? And will it make any difference to the current 
flowing through A? 

Let us see. The circuit B having the same resistance 
as circuit A conducts just as well, hence twice as much 
current can flow between C and D, as flowed before. It is 
evident that the resistance offered is now half what k 



ohm's Law 



295 



was. In Fig. 141 it was 50 ohms, in Fig. 142 it must be 
25 ohms, by Ohm's law the current through the dynamo 

has doubled, for C= =f-= =4 amperes; 2 amperes 

K 25 

through A, and 2 amperes through B. 

To make this quite clear, let us take our water analogy 
again. Fig. 143 represents a water circuit similar to our 
last electrical circuit. If the pump be working con- 
tinuously, maintaining a difference of pressure between 
its ends, then with T turned on and t turned off, we 




X&ffiSS^ 



Fig. 141. A Series Circuit. 



should have a flow of water round the circuit through A, 
which would offer the principal resistance of the circuit, 
and the current gauge would indicate a certain current 
flowing through the pump. If now we turn on tap t, we 
open up another path for the water to flow in, and con- 
sequently, as water will flow in B just as easily as in A, 



296 



ELEMENTARY ELECTRICITY 



the resistance to the passage of water from C to D would 
be halved, and the current gauge would immediately in- 
dicate twice the former current. The two pipes in paral- 
lel are really equivalent to one pipe of twice the internal 
sectional area. The same thing would apply to 3, 4, io, 
or any number of similar pipes joined between C and D ; 
the resistance would be reduced to j£, J4> or 1/10 its 
former value, with a corresponding increase in the cur- 
rent flowing through the pump. 




V&sSS^ 



Fig. 142. Parallel Circuits. 



It must be understood that there is no increase in 
the current flowing through any individual pipe when 
others are connected across. The current in A, Fig. 
143, for instance, would remain practically constant 
throughout providing the pump maintained the same 
difference of pressure. 

It is in this way that we must look upon the electrical 
current in Fig. 142. The more similar wires we join 
between G and D, the more are we increasing the con- 






PARALLEL CIRCUITS 



297 



ductivity between these two points, and the less is the 
resistance becoming, but providing the dynamo main- 
tains the pressure, no alteration would take place in the 
value of the current in any individual conductor. Each 
separate conductor would act according to Ohm's law, 

| and each being joined to points, maintained the same 
difference of potential, and each being of the same re- 

| sistance, each must have the same strength of current 
flowing through it. 




Fig. 143. Elydraulic Circuit with. By-pass. 



Effects of Current. 

Imagine a circuit like Fig. 144 containing in order 

1. A galvanometer or current meter. 

2. An electromagnet. 

3. An apparatus for electrolysing water or a gas volt- 
ameter. 

4. A copper plating bath or copper voltameter. 



298 



ELEMENTARY ELECTRICITY 



5. A vessel containing a coil of wire submerged in 
water, and surrounded with a box of sawdust to pre- 
vent the radiation of the heat. 

6. An incandescent lamp. 




£f«cCl...wj~^ &~" %&a~~**«>' 



1i>tUL**x*C4*r 




Fig. 144. The Different Effects of Current " 



When the current is flowing you will observe the fol- 
lowing effects : 

1. The needle is deflected. 

2. Armature of magnet is attracted and held. 

3. Water is turned to oxygen and hydrogen gases, 

4. The cathode is copper plated. 

5. The thermometer rises. 

6. The lamp gives light. 



EFFECTS OF CURRENT 299 

Now let us examine the size of each of these effects, 
measuring them in the most suitable units, and then let 
the current be changed from its original value to say 
twice and then three times that value and see how the 
size of the effect varies. 

The Galvanometer gives readings of 10, 15, 17 de- 
grees, so that the effect of a given current varies ac- 
cording to whether it is the only current passing or one 
of many equal currents. 

The Electro-magnet gives 9 lbs. pull, then 11 lbs., and 
finally 1 1^2 lbs.; so that evidently the attraction of a 
magnet does not increase in the same proportion as the 
increase of current in its coils. 

In the Gas Voltameter we find that double the current 
electrolyses double the quantity of water, and three times 
the current, treble the water. The same is true of the 
Copper Plating Bath. The amount of metal deposited 
is exactly proportional to the current and the time. A 
given current deposits 0.0026 pounds of copper per hour ; 
double the current will deposit 0.0052 pounds, treble the 
current 0.0078 pounds. 

The thermometer in the Calorimeter has risen very 
rapidly. The original current made it rise 1/10 of a 
degree a second, twice the current gave a rise of 4/10 
degrees and thrice the current gave 9/10 degrees rise. 

Since I x 1 equals 1 
and 2x2 " 4 
and 3 x 3 " 9 

And since the numbers 1, 2 and 3 represent the cur- 
rents; and 1, 4, 9 the heat produced, we say that the 
heating is proportional to the square of the current. 



300 ELEMENTARY ELECTRICITY 

(The product obtained by multiplying a number by itself 
is called the square of that number). 

The extra amount of Light obtained by increasing 
the current is small and does not increase regularly. 
The first increase in current adding 3 candle power, and 
the next increase added 4 candle power, while the next 
would have probably added 6 candle power. 

Looking back over these current effects it will be seen 
that the electrolysis or electro-chemical effect is the only 
one that is regular in a simple manner, so we choose the 
amount of copper or silver plating done as the test for 
the size of the current flowing. We decide that the unit 
of current shall be called an 

AMPERE, which is the current depositing 0.0026 
pounds of copper or 0.00887 pounds of silver per hour. 
Although a certain current may be lighting a lamp, if 
that current were made to silver plate and deposited 
0.00443 pounds in y 2 hour, we should call it an ampere. 

The original Edison Meters were nothing but zinc 
plating baths put into the customers line and by the 
amount of plating done the amount of current drawn by 
the lamps was known. 

Modern meters are either galvanometers marked to 
show amperes or are small motors with a cyclometer 
attachment to record the ampere-hours. 

An ampere-hour is one ampere for one hour, half an 
ampere for two hours, etc 



LESSON 18. 
Resistance. 

Every conductor offers more or less opposition to the 
flow of electricity and we call the opposition Resistance. 

In order to compare the resistance of different ma- 
terials some standard of resistance must be agreed upon. 
All electricians and engineers have settled on the 

OHM. The unit of resistance is the ohm, which is 
the same resistance as is offered by a mercury conductor 
made in this way: Take 0.0318 of a pound of pure 
mercury and pour it into a glass tube which is exactly 
41.88 inches long, and of uniform size of bore. The 
mercury must exactly fill the tube when both are at a 
temperature of 32 degrees Fah. Try different sized 
tubes of the same length (41.88 inches) until one is 
found to be exactly filled. Then the resistance offered 
by this conductor to the passage of electric current is 
called one ohm. Remember that the temperature must 
always be 32 degrees. 

Mercury is selected because it is a liquid and because 
its resistance is high and so the tube is not inconveniently 
long. 

To measure all resistances the ohm is used, but to 
express very small resistances the microhm is used. It 
is one millionth of an ohm. 

To express very high resistances the megohm is used, 
It is one million ohms. It is abbreviated meg. 

301 



302 ELEMENTARY ELECTRICITY 

A telephone engineer would refer to a resistance of 
O.00385 ohms as 3850 microhms. Ohms X 1,000,000== 
microhms. 

In speaking of the resistance between the two track 
rails measured across the ties an engineer would proba- 
bly say 47.5 megs instead of 47,500,000 ohms. 

Ohms-f- i,ooo,ooo=megohms. 

Laws of Resistance. 

The resistance varies with the nature of the material. 
The metals are good conductors, cotton and dry goods 
very poor conductors, while silk, porcelain, shellac, oils, 
mica, paraffin, and dry air are so poor that we call them 
insulators. 

If we take a piece of wire having a known resistance, 
and cut it into two equal lengths, we find on measuring 
that each piece has just half the resistance of the former 
piece, hence the law is twice the length, twice the re- 
sistance. 

Stated mathematically it is — 

The resistance of a given wire of uniform section is 
proportional to its length. 

But the resistance also depends on the cross section 
of the wire. If we take three wires of the same ma- 
terial, but with cross sections, of I, 2 and 3 circular 
mills,* if the first one has 1 ohm resistance, the others 
will have Yz and J^ of an ohm resistance. 

Hence the greater the area of wire's cross section the 
less the resistance. 

The resistances are inversely proportional to their 
cross sectional areas. 



*See further on in Lesson. 



RESISTANCE 303 

It must be remembered that the cross sectional area 
is obtained by squaring the diameter. If the diameters 
of two wires are 3 and 6 mils, their areas are 9 and 36 
circular mils and the resistance of the first wire is 4 
times as great as the second, because the area is only 
Y$ of the second. 

The resistance of conductors also varies very largely 
with the nature of the material used. For instance, if 
we take three different wires, all the same length, and 
the same sectional area, but one made of copper, an- 
other of iron, and the third of german silver, their re- 
sistances would be in the ratio of 1:6:13. It is useful 
to remember these figures as being approximately cor- 
rect, for the three metals named are in great demand 
in electrical engineering. 

German silver made of 2 parts copper, 1 of zinc and 
I of nickel, is an extra high resistance metal, and is 
useful when we need a lot of resistance in a small space. 
The resistances used to aid in the starting of traction 
motors are usually of cast iron. 

Stated briefly, the longer the piece of material the 
higher is the resistance, and the greater its cross section 
the lower the resistance. The rule being: 

^ . . . Material x Length 

Resistance in ohms= — — -= — r- 5 — 

Cross bection. 

The number representing the materials are the ohms 
resistance of a piece 1 foot long and 1 mil in diameter. 

Copper 10.8 Iron 63.4 

Aluminum 17.2 Mercury 128.3 
German Silver 586.2 



304 ELEMENTARY ELECTRICITY 

For Length insert the number of feet and for Cross 
Section put the square of the diameter in mils (thou- 
sandths of an inch). 

For example: Find the resistance of 9000 ft. of iron 
wire 0.2 inch in diameter. 

Material: Iron 634. Length in feet 9000 

Diameter in mils 200. Squared 40000 

_ . , 6^.4 x QOOOO , 

Resistance in ohms= - — — £: = 14.26 

40000 

This rule only applies to round wires but may be 
changed so as to apply to rectangular conductors. 

" . Material x Length 

Resistance^ : 2— 

Areax 1.27 

Area being the area of the cross section of rod in 
square mils. 

An increase in the temperature increases the resist- 
ance of the metals, but they each have their own way 
of increasing, some faster than others. 

The resistance of copper increases a little less than % 
of 1% for even- degree Fahr. and iron increases a little 
more than copper. Mercury increases about 1/40 of 
1% per degree. 

Wire Measurement. 

The diameter of a round wire or bar is always meas- 
ured in mils. A mil is a thousandth of an inch. 

The area of round wires is measured in Circular mils. 
The number of circular mils is found by squaring the 
diameter measured in mils. 



RESISTANCE 305 

This is a far better way of measuring than the me- 
chanics way of square inches, but cannot be applied to 
rectangular pieces of material. 

To make matters as simple as possible the electrician 
measures the two dimensions of the cross section in mils 
and obtains the area (by multiplying them together) in 
square mils. This he converts at once to circular mils 
by multiplying by 1.27. Then having circular mils he 
can apply formulas* 

The mil-foot is used in formulas and is handy as a 
comparison of resistances. 

A piece of round material 1 foot long and I mil in 
diameter is a mil-foot. 



Resistance and Conductivity. 

If we take a piece of wire whose resistance is 1 ohm, 
and apply an E. M. F. to the ends of it, we find that it 
is able to conduct electricity. We might say that this 
piece of wire has unit conducting power, or unit con- 
ductivity, as well as unit resistance. If we take another 
piece of the same wire, but twice the length of the former 
piece, it will have twice the resistance, that is, it will con- 
duct electricity only half as well as the former piece, con- 
sequently we should say this piece of wire has resistance 
of 2 ohms and a conductivity of y 2 . Again, if we take a 
wire having a resistance of 10 ohms, then it will conduct 
electricity only 1/10 as well as the piece having 1 ohm. 
Therefore we should say its conductivity is 1/10, and so 
on. 

We thus see that the conductivity of a substance is the 
reciprocal or the reverse of its resistance. If the resist- 



306 ELEMENTARY ELECTRICITY 

TABLE OF DIMENSIONS OF PURE COPPER WIRE.* 










Area. 


Weight and Length, 
Sp. Gr. 8.9. 


No. 


Diam. 

Mils. 












B. &S. 


Circular 


Square 


Lbs. per 


Lbs. per 


Feet per 






Mils. 


Mils. 


1000 feet. 


Mile. 


Pound. 


0000 


460.000 


211600.0 


166190.2 


640.73 


3383.04 


1.56 


000 


409 640 


167805.0 


131793.7 


508.12 


2682.85 


1.97 


00 


364.800 


133079.0 


104520.0 


402.97 


2127.66 


2.48 





324.950 


105592.5 


82932.2 


319.74 


1688.20 


3.13 


1 


289.300 


83694.5 


65733.5 


253.43 


1338.10 


3.95 


2 


257.630 


66373.2 


52129.4 


200.98 


1061.17 


4.98 


3 


229.420 


52633.5 


41338.3 


159.38 


841.50 


6.28 


4 


204.310 


41742.6 


32784.5 


126.40 


667.38 


7.91 


5 


181.940 


33102.2 


25998 4 


100.23 


529.23 


9.98 


6 


162.020 


26250.5 


20617.1 


79.49 


419.69 


12.58 


7 


144.280 


20816.7 


16349.4 


63.03 


332.82 


15.86 


8 


128.490 


16509.7 


12966.7 


49.99 


263.96 


20.00 


9 


114.430 


13094.2 


10284.2 


39.65 


209.35 


25.22 


10 


101.890 


10381.6 


8153 67 


3144 


165.98 


31.81 


11 


90.742 


8234.11 


6467.06 


24.93 


137.65 


40.11 


12 


80.808 


6529.94 


5128 60 


19.77 


104.40 


50.58 


13 


71.961 


5178.39 


4067.07 


15.68 


82.792 


63.78 


14 


64.084 


4106.76 


3225.44 


12.44 


65.658 


80.42 


15 


57.068 


3256.76 


2557.85 


9.86 


52.069 


101.40 


16 


50.820 


2582.67 


2028.43 


7.82 


41.292 


127:87 


17 


45.257 


2048.20 


1608.65 


6.20 


32.746 


161.24 


18 


40.303 


1624.33 


1275.75 


4.92 


25.970 


203.31 


19 


35.890 


1288.09 


1011.66 


3.90 


20.594 


256.39 


20 


31.961 


1021.44 


802.24 


3.09 


16.331 


323.32 


21 


28.462 


810.09 


636.24 


2.45 


12.952 


407.67 


22 


25.347 


642.47 


504.60 


1.95 


10.272 


514.03 


23 


22.571 


509.45 


400.12 


1.54 


8.1450 


648.25 


24 


20.100 


404.01 


317.31 


1.22 


6.4593 


817.43 


25 


17.900 


320.41 


251.65 


.97 


5.1227 


1030.71 


26 


15.940 


254.08 


199.56 


.77 


4.0623 


1299.77 


27 


14.195 


201.50 


158.26 


.61 


3.2215 


1638.97 


28 


12.641 


159.80 


125.50 


.48 


2.5548 


2066.71 


29 


11.257 


126.72 


99.526 


.38 


2.0260 


2606.13 


30 


10.025 


100.50 


78.933 


.30 


1.6068 


3286.04 


31 


8.928 


79.71 


62.603 


.24 


1.2744 


4143.18 


32 


7.950 


63.20 


49.639 


.19 


1.0105 


5225.26 


33 


7.080 


50.13 


39.369 


.15 


.8015 


6588.33 


34 


6.304 


39.74 


31.212 


.12 


.6354 


8310.17 


35 


5.614 


31.52 


24.753 


.10 


.5039 


10478.46 


36 


5.000 


25.00 


19.635 


.08 


.3997 


13209.98 


37 


4.453 


19.83 


15.574 


.06 


.3170 


16654.70 


38 


3.965 


15.72 


12.347 


.05 


.2513 


21006.60 


39 


3.531 


12.47 


9.7923 


.04 


.1993 


26487.84 


40 


3.144 


9.88 


7.7635 


.03 


.1580 


33410.05 



* 1 mile pure copper wire 
1-16 inch. diam. ; 



13.59 ohms at 15.5° C. or 
~ 59.9° F. 

1 circular mil is .7854 square mil. 



r' 



RESISTANCE 307 

ance of a conductor be 50 ohms, its conductivity is 1/50. 
A name has been given to the unit of conductivity which 
is easy to remember. Seeing that the conductivity is the 
reverse of resistance, the name of the unit of resistance 
(ohm) has been reversed for that of conductivity. Thus 
a wire of 1 ohm resistance has 1 mho conductivity. A 
wire of 75 ohms resistance has a conductivity of 1/75 
mho, while a wire of J/£ ohm resistance has a conductiv- 
ity of 2 mhos. 

Of course it will be understood that if conductivity is 
the reciprocal of resistance, then resistance is also the re- 
ciprocal of conductivity, one the reverse or reciprocal 
of the other. Therefore a wire of 1/50 mho conductiv- 
ity has a resistance of -j- = 50 ohms. 

Resistances in Series and in Parallel. 

When resistances are in series we add their values to- 
gether to get the total resistance but when they are in 
parallel the resistance of the group, called joint resist- 
ance, is less than the smallest and must be calculated in 
a certain manner. 

Turn back to Fig. 142. A can conduct electricity 
across between A and D its conductivity being 1/50 mho, 
that is, it will conduct electricity across only 1/50 as well 
as a resistance of ohm. But we have now got two paths, 
each with a conductivity of 1/50 mho, so the two to- 
gether can conduct electricity across twice as well as one 
of them, for now we have a conductivity of 1/50+ 1/50 
=1/25 mho. We have already seen that resistance is the 
reciprocal of conductivity, therefore the resistance be- 
tween C and D is now -7- =25 ohms. But it was 50 

25 



308 



ELEMENTARY ELECTRICITY 



chms before we joined the second wire across, so that w£ 
have reduced the resistance to half its former value. 

If there should be resistances in series with those in 
parallel figure the joint resistance of the set in parallel 
and then figure the rest as if that joint resistance took the 
place of the parallel group and everything were in series. 

Consider Fig. 145. Here we have C and D joined by 
two wires, A having a resistance of 50 ohms, and B hav* 
ing a resistance of 25 ohms. 




Fig. 145. A Divided Circuit. 



Now we have seen that A has a conductivity or con- 
ducting power=i/50, similarly B has a conductivity= 
1/25, and therefore the two together have a conductivity 
of 1/50+1/25=3/50 mho. The resistance between C 
and D being the reciprocal of the conductivity 



is =— = -—=16.0 ohms 
A 3 



(less than the smallest resistance). 






RESISTANCE 309 

The current flowing through the dynamo now is— 

r E 100 , 
C= R = i^6 =6amp€reS ' 

Again, imagine C and D to be connected by three 
wires, A=5o ohms, B=2$ ohms, and G=io ohms. Then 
the conductivity between 

r a t^ 1 1 1 1 + 2 + 5 8 « 
C and D =-+ -+ To = -^-= - mho, 

and the resistance between 

C and D ==— = -—=6.25 ohms 



_8_ 
50 



8 



(again less than the smallest resistance joining C and D). 

Of course the combined resistance -must be less than 
that of the smallest resistance between the points, for if 
G were there alone, that part of the circuit would have 10 
ohms resistance, and the addition of B and A, though 
larger than G, is only diminishing the resistance between 
these points by opening up other paths for the passage of 
electricity. 

Q. 1. What is the resistance of four wires in par- 
allel of 2, 5, 10, and 20 ohms respectively? 

The combination has a conductivity of — 

1,1,1,1 10+4+2 + 1 17 , 
"2 + T + 10 + 20 = 20 = 20 mh °' 

and their combined resistance in parallel 
1 20 ." " 

= JTT7 0hms ' 



310 ELEMENTARY ELECTRICITY 

Q. 2. Two mains are carrying current for a group of 
twenty lamps; each lamp has a resistance when incan- 
descent (white hot) of 160 ohms, and they are all joined 
in parallel. What is the resistance between the two 
mains. 

Here all the resistances are equal and the total resist- 
ance is 1/20 of one of them or 160-^-20=8 ohms. 

If we have only to deal with two resistances in parallel, 
it will be easier and quicker to make use of this rule : 

The joint resistance of two resistances in parallel is 
the product of the two divided by their sum. 

Q. 3. Two resistances of 5 ohms and 20 ohms are in 
parallel. What is their combined resistance? 

_, product 5x20 100 . 



Working by the first method we have : 
Conductivity=4 + ^=! 



20 
Resistance=— =4 ohms. 
o 



LESSON 19. 

Ohm's Law. 

We have now stated what an ampere of current and 
an ohm of resistance are. The unit of pressure is the 
result of these two, for a volt is the pressure necessary 
to send one ampere of current through one ohm of resist- 
ance. 

Knowing these three units of measurement and the 
law of flow of current we can solve many electrical 
problems. 

Law of the Flow of Current. 

With a given circuit the greater the pressure the 
greater the current, or if the pressure (voltage) re- 
mains the same, the less the resistance the more current 
flows. 

Hence Current equals Pressure divided by Resistance, 

. volts 

or Amperes= — : 

ohms 

This is the regular form of Ohms Law and means: 
The current in amperes in any conductor is equal to 
the difference in pressure between the ends of the con- 
ductor, in volts; divided by the resistance between the 
ends, expressed in ohms. 

311 



312 ELEMENTARY ELECTRICITY 

Expressed as a formula, Ohms Law is 
r E* V n PD 

c= iT orC== R or c= lf 

By E we mean the E. M. F., or electromotive force, 
that is the total pressure in the circuit measured in volts. 

By V we mean the pressure (in volts) in the part of 
the circuit we are considering. 

By P. D. we mean the Pressure Difference or differ- 
ence in pressure (in volts) between the ends of the part 
of the circuit we are considering. 

It is evident that V and PD are the same, while E is 
the sum of all the V's in the circuit. 

The form V=CR means: 

(i) The voltage required to maintain a current flow 
of C amperes through R ohms is given by the product of 
C and R. 

(2) The drop of pressure or voltage lost in any con- 
ductor is equal, to the product of the current C and 
resistance R. 

In fact, the loss in pressure which always occurs when 
transmitting current is often called the CR loss. The 
"drop" is the usual term. 

E 
The form R= ^— is used to find what resistance 

must be used in connection with a pressure E to limit 
the current to C amperes. 



*Electrical magazines and many text books use I for current, 
reserving C for capacity. It will soon be generally adopted by 
every one ; but for a student C is more convenient and expressive. 






ohm's law 313 

Problem i. An incandescent lamp having a resist- 
ance when hot of 240 ohms is connected to mains having 
120 volts pressure between them. How much current 
does the lamp draw? 

n v 120 1/ 

^R = 240 =/2ampere ' 

Problem 2. What pressure will be required to force 
7 amperes through an arc lamp whose resistance hot is 
7 ohms? 

V=CR=7X7=49 volts. 

Problem 3. What drop will there be in transmitting 
2000 amperes to a locomotive 2 miles from power house, 
with a circuit whose resistance is 1/20 of an ohm per 
mile? 

2 miles=2/20=i/io ohms. 

Drop=V=CR=2oooX 1/10=200 volts. 

Problem 4. In a car heater enough heat is generated 
when 10 amperes are flowing. Five of them are to be 
placed in series in a car. Voltage between third rail 
and track 500. What must be the resistance of the 
heater when hot? 

500 volts-^5 heaters=ioo volts per heater. 

R=~=— =ioohms. 

Problem 5. In Fig. 146 let the dynamo of 0.01 ohms 
resistance be producing 100 volts as measured on the 
volt meter V. This is not the E. M. F. of the dynamo, 
because there is some drop in the dynamo. The 100 



314 



ELEMENTARY ELECTRICITY 



volts is the V. or P. D. at the ends of the external cir- 
cuit. 

This external circuit contains resistances as follows: 
A in series 2 ohms. C and E together in parallel., yet 
in series as a group with A. C is ioo ohms. E is 300 
ohms. B is 3 ohms in series with A and the parallel 
group. 




Fig. 146. Dynamo in a Series — Parallel Circuit 

What current flows through the dynamo? 

The same current as through A or B for the circuit 
is a series one. The resistance of external circuit is as 
follows : 



A= 



2 ohms 



~ , -^ . . ,, product ^0000 , 

C— E jointly= = — =73 ohms 



sum 



B= 



400 

3 ohms 

Total=8o ohms 



A circuit of 80 ohms has 100 volts pressure at its 
ends, therefore, 

r V 100 Q 

C=-=— =1.25 ampere flows. 



ohm's law 315 

The drop in the dynamo must be 

V=CR=i. 25X0.01=0.01 25 volts. 

A trifling amount, it is true, but should the dynamo de- 
liver 1000 amperes the drop becomes 10 volts, which is 
large enough to be considered. If the voltmeter were 
placed around A what would it read? It would read 
the drop in A. 

01=1.25X2=2.5 volts. 

Grouping Cells or Dynamos. 

A cell or dynamo is a source of E. M. F. and is in 
addition a source of resistance. 

When we wish a higher voltage than one cell on ma- 
chine will give, we connect several in series. This in- 
creases the voltage and resistance and the result depends 
on the resistances of the external and internal circuit. 
The part of circuit in the cells or dynamos is the internal 
circuit. 

Problem 1. Six blue stone cells, each 1 volt E. M. F. 
and 3 ohms resistance, are in series on a 100 ohm exter- 
nal circuit. Add 6 more in series. Will the current be 
doubled? Ri and Re are abbreviations for internal and 
external resistances. 

Cell Ri= 3 ohms 
6 cells Ri= 18 ohms E M F=i volt 
Re=ioo ohms E M F=6 volts 



R=n8 ohms 



C== R = il8 =0 '° 5 ( near1 ^ am Peres. 



316 




ELEMENTARY 


l ELECTRICITY 


Add 6 


more 










Ri= 


36 ohms 


E M F= 


--12 volts 




Re= 


100 ohms 










136 ohms 






C= 


=12/136=0.09 


(nearly) 


amperes. 



Answer: The current is almost doubled. 

Problem 2. The same cells as in Problem 1 are con- 
nected to external circuit of 1 ohm and 6 more cells are 
?:Med in series. Is the current doubled? 

6 cells Ri=i8 ohms EMF=6 volts. 

Re= 1 

R=i9 








E 

~R~ 


6 
= — =0.12 
19 ° 


(nearly) amperes. 


Adding 6 1 
Rfc 

Re= 


n series 
=36 ohms 
= 1 ohm 


EMF=i2 volts 


R= 


=37 


ohms 






c= 


E 12 

-= — ;=o.32+amperes. 



Answer. Xo. Practically no increase of current. 

Moral: With high external resistance add more E. 
M. F. in series to increase current (the added resistance 
does no harm). With low external resistance add noth- 
ing in series unless it has a very low internal resistance. 



OHMS LAW 317 

Suppose in Problem 2 we had added 3 storage bat- 
teries at 2 volts and 0.33 ohms each. 

Old Ri=i8 Old EMF=6 

New Ri= 0.99 New EMF=6 

Re= 1. 



E=I2 volts 



R=2o ohms 



r E 12 . 
C= R=20= a6ampereS - 



Which is practically double the previous current. 



i* '""' JT 


1 


■1 m 




m m 


| 


K* <-. 


A 



Fig. 147. Batteries in Parallel. 



In Problem 2 had there been nothing available except 
6 more blue stone cells they should have been put in 
parallel with the others, similar to Fig. 147. 

Each set of 6 cells has Ri=i8 but joint resistance of 
two groups is 9 ohms. 

Ri= 9 ohms 
Re= 1 ohm 



R=io ohms 
The E. M. F. of each set of 6 cells is 6 volts and 



318 ELEMENTARY ELECTRICITY 

the E. M. F. of the two groups not adding together 
makes E. M. F. of group, 6 volts as before. 

E 6 r 

-= Jq= - 6 amperes, 



which is practically double the previous current. Hence, 
when external resistance is low, lower your internal re- 
sistance by adding more cells in parallel. 

There is a silly rule : 

The best arrangement of cells is when the internal 
and external resistances are equal. 

This is an arrangement to force the battery to deliver 
the greatest possible current. The efficiency will be 50% 
because since Ri and Re are equal the drop in each is 
the same, hence half the pressure is doing useless work 
and half useful work. 

For economy have internal resistance low as compared 
with external resistance. 

When a battery is at work on a high resistance 
line, add cells in series to increase current. When ex- 
ternal resistance is low always add cells in parallel. 

These rules do not apply to most dynamos because 
their internal resistance is very low. 

With dynamos to get more voltage place extra ma- 
chines in series. You will then get more current also. 

To get more current at same voltage place extra ma- 
chines in parallel with the first one. 



THE INDUCTION COIL. 

Induced electric currents have in general very high 
electro-motive forces, and are able to spark across spaces 
that ordinary battery currents cannot possibly cross. In 
order to observe these effects, a piece of apparatus in- 
vented' by Mason, and improved by Ruhmkorff, and 




Fig. 148. 



termed the induction coil or inductorium (see Figure 
148), is used. The induction coil consists of a cylindrical 
bobbin having a central iron core surrounded by a short 
inner or "primary" coil of stout wire, and by an outer 
"secondary" coil made up of many thousand turns of 
very fine wire, which is carefully insulated between its 
different parts. The primary circuit is connected to the 

319 



320 ELEMENTARY ELECTRICITY 

terminals of a few Grove's or Bunsen's cells, and in it 
are also included an interruptor, and a commutator, or 
key. The object of the interruptor is to make and break 
the primary circuit in rapid succession. The result of 
this is at every "make" to induce in the outer "second- 
ary" circuit a momentary inverse current, and at every 
"break" a powerful momentary direct current. The cur- 
rents at "make" are suppressed, as explained further 
on; the currents at "break" manifest themselves as a 
brilliant torrent of sparks between the ends of the sec- 
ondary wires when brought near enough together. The 
primary coil is made of stout wire, that it may carry 
strong currents, and produce a powerful magnetic field 
at the center, and is made of few turns to keep the 
resistance low, and to avoid self-induction of the primary 
current on itself. The central iron core is for the pur- 
pose of increasing, by its great coefficient of magnetic 
induction, the number of lines of force that pass through 
the coils. It is usually made up of a bundle of fine 
wires in order to avoid the induction currents which 
would be set to circulating in it if it were a solid bar, 
and which would retard its rapidity of magnetization, or 
demagnetization. The secondary coil is made of many 
turns, in order that the coefficient of mutual induction 
may be large, and as the electro-motive force of the in- 
duced currents will be thousands of volts, its resistance 
will be immaterial, and it may be made of the smallest 
size wire that can be conveniently wound. Induction 
coils have been constructed which will yield a spark 42 
inches in length in the air when worked with .30 Grove's 
cells. In an induction coil of this capacity the secondary 
coil would contain 280 miles of wire, wound on in 340,000 
turns, and having a resistance of 100,000 ohms. 



THE INDUCTION COIL 



321 



The interruptors of induction coils are usually self- 
acting. That of Foucault, shown with the coil in Figure 
148, consists of an arm of brass L, which dips a platinum 
wire into a cup of mercury M, from which it draws the 
point out, so breaking circuit, in consequence of its 
other end being attracted toward the core of the coil 
whenever it is magnetized. 



ELECTRO-MAGNETIC INDUCTION. 

If an electric conductor lies in a field of force (it may 
be in the vicinity of a magnet pole), it will remain un- 
affected by the field, in so far as any electro-motive force 
in it is concerned, so long as it is not moved; but if the 
conductor is moved so as to cut the lines of force, or 
if the magnet is moved, while the conductor is station- 
ary, which brings about the same result of cutting lines 
of force, a certain amount of electro-motive force will 
be impressed upon the conductor. 

There are many variations in the relations of conduc- 
tors and fields of force which have the effect of impress- 
ing electro-motive force upon such conductors, and pro- 
ducing currents in them, provided the conductors form, 
or are part of, a closed circuit. Generally speaking, 
these inductive effects involve attraction, or repulsion be- 
tween magnet pole and conductor. 

There are two general methods of construing the ac- 
tion or influence of a field of force upon a moving con- 
ductor. It may be referred to cutting of lines of force 
by the conductor, or to changing the number of lines of 
force which pass through the space included in the elec- 
tric circuit. The latter may be looked upon as a ring, or 
irregular circle-like lead of wire. The passing of lines 
of force through this circle of wire is often called thread- 
ing or interlinking of lines of force. The latter expres- 
sion is correct, because lines of force form closed cir- 
cuits of their own. 

Induction. — When an electric conductor forming part 
of a circuit is swept through a field of force an electro- 

322 



ELECTRO-MAGXETIC INDUCTION 



323 



motive force is impressed upon it. If the ends of r be 
conductor were connected to a proper instrument, such 
as a voltmeter, the electro-motive force would affect its 
index, and it would be evident that electro-motive force 
actually existed. The cutting of lines of force by an 
electric conductor represents the impressing of force 
upon or transferring of force to the conductor. The 
term force, as last used, applies to electro-motive force. 
If the proper conditions are established, the electro- 



A 




n 



y^^ 



Fig. 149. Ring Moving in Field of Force Without Cutting Lines of Force. 



motive force impressed on the conductor by the field o 
force will produce a current. If these conditions do 
not exist, no current will be produced. Thus there are 
two varieties of induction. In the one case energy in the 
form of volt-coulombs, or other electro-motive force- 
quantity unit, is developed, and by the law of the con- 
servation of energy the motion of the conductor through 
the field of force is resisted, so that energy has to be 
expended upon it to move it across the lines of force. In 
the other case no current is produced, and no energy is 
required to move an open-circuit conductor through the 
field. 



324 



ELEMENTARY ELECTRICITY 



Conditions for Inducing Electric Energy. — The condi- 
tions for thus producing current are two. The conductor 
must form part of a closed circuit, and the number of 
lines of force passing through the loop or opening of 
the circuit must vary in number ; or a portion of the cir- 
cuit must cut lines of force. In most cases of dynamo 
generators both the latter conditions exist at once. As 
the armature conductors cut lines of force they vary 
the number of lines of force interlinked with the circuit. 






ft 



u 




Fig. 150. Ring Moving in Field of Force Cutting Lines of Force Without 
Change of Interlinked Lines. 

Examples of Interlinking. — Assume a uniform field of 
force and let a ring of conducting material be moved in 
it. The cuts, Figures 149 to 152, illustrate several con- 
ditions, the motion of the ring being indicated by the 
arrows. 

In the case illustrated by Figure 149 the ring is swept 
through the field of force, but cuts no lines of force, as 
its motion is parallel to them. Therefore no electro- 
motive force is § impressed upon it. In the case shown 
in the next cut, Figure 150, lines of force are cut, there- 
fore electro-motive force is impressed; but as the num- 






Electro-Magnetic induction 



325 



ber of lines of force embraced in the ring is unchanging, 
ho current is produced. Each half of the ring has electro- 
motive force of the same polarity impressed oh it and 
the two oppose each other, so that no current results. 
Ill Figure 151 the ring is swung around so that it not 
only cuts lines of force, but the number of lines em- 
braced by it is constantly varying, hence electro-motive 1 
force and current both result. In the riext cut, Figure 
152, the ring is swept in a straight line through a non- 
uniform field of force; It not only cuts lines of force* 



ft 



* 1 ~ v : s 

< ' ^ ■ 

< — W — / -^ 




Fig. 151. Ring Moving in Uniform Field of Force Under Conditions Producing 

a Current. 



but the number passing through it varies constantly. 
Electro-motive force and current both are produced. In 
the first two cases no power is expended on moving the 
ring through the field; in the last two, power is so ex- 
pended. 

Motionless Conductor in a Field of Force of Varying 
Density. — Where a ring or convolution of wire or other 
conductor is placed in a magnetic field, lines of force 
will pass through it, if its plane of position is at an angle 



^26 



ELEMENTARY ELECTRICITY 



to the general direction of the lines of force. Lines of 
force would be said to thread through it, but would have 
no effect whatever upon it. It has been seen that a cur- 
rent would flow through it, actuated by electro-motive 
force, if the wire were moved so as to vary the number 
of lines of force embraced by the circuit. Suppose the 
wire or conductor to be kept motionless, and the density 
of the field of force to vary. This would cause the 
lines of force embraced by the circuit to vary in num- 
ber. Electro-motive force and current would be pro- 
duced in the conductor exactly as if it were moved. 




Fig. 152. 



Ring Moving in Field of Force Under Conditions Producing a 
Current. 



Energy Relations. — Energy would be absorbed, wheth- 
er the field of force was increased or diminished in density 
under the above conditions. The presence of the closed 
circuit would be the cause of such expenditure. It 
would by counter-electro-motive force resist any change 
of field density which would produce energy in its con- 
ductor, and exact the expenditure of additional energy. 



ELECTROMAGNETIC INDUCTION 32? 

Fields of Force in Practice. — In practical engineering 
"fields of force are produced by magnets, which are gen- 
erally electro-magnets. They vary in the number of 
their poles, but follow pretty closely some general rules. 
The poles are nearly always of even number — that is, 
for every north pole there is a south pole. The north 
and south pole are placed in alternation with each other. 
Fields of force may be moved past conductors, or coils 
forming parts of circuits, or the conductors and coils 
may be moved past the fields of force. Again, the rela- 
tions of field to conductors may be kept changing, as 
in the case of inductor generators. In all such cases 
electro-motive force is imparted to the circuit. The con- 
ductors or coils which are thus treated form parts of 
armatures, in fact, they constitute the active portions of 
the armature windings. The effect of these processes 
is to cause the number of lines of force interlinked with 
the circuit to vary. 

Direction of Current Induced by Cutting Lines of 
Force. — If the north pole of a horizontally placed mag- 
net face the observer, the lines of force will come ot|t 
of it toward him, will curve around and pass through 
the space surrounding the pole away from it to the south 
pole. If a perpendicular conductor is swept from left 
to right across the north pole, an electro-motive force will 
be induced in it, tending to produce in it a current from 
above downward. Let a letter N be marked upon the 
pole. Rule lines upon the end parallel to the oblique 
stroke of the N. Cut a narrow slit in a card and, hold- 
ing it with the slit vertical, move it to right or to left. 
The lines will appear through the slit, like a series of 
dots, and will appear to move up or down — up for a 
motion to the left, down for a motion to the right. Their 



§28 



ELEMENTARY ELECTRICltY 



apparent motions indicate the direction of currents in- 
duced in a vertical conductor moved across the north 
pole to left or right. The cut, Figure 153, illustrates the 
principle. In it the south poles are diagonally shaded in 
the opposite direction to the north poles. 

The sarhe process of using a slotted card will show 
the direction of currents in a cohductor moved across 
therri. In Figure 153 the arrows a b and c d indicate 
the direction of flow of the current induced by triotion 



V" 



<v— > 




Fig. 153. Directions of Induced Currents. 



in the direction of the horizontal dotted arrows. If the 
motion were in the opposite direction, the currents would 
-have the reverse directions. Two causes may be as- 
signed for electro-magnetic induction. First, the cut- 
ting of the lines of force by conductors wound upon a 
drum. This generally has the result of changing the 
number of lines of force threading the circuit. Second, 
changing the number of lines of force threading the cir- 
cuit, without reference to cutting them by conductors. 






ELECTRICAL MEASURING INSTRUMENTS. 

The ll'hcatstonc Bridge is an apparatus for determin- 
ing the resistance of a conductor. 

If a conductor carrying a current is divided into two 
parallel conductors for a portion of its length, the fol- 




Fig. 154. Diagram of Wheatstone Bridge. 



lowing law will always exist : For every point on one of 
the parallel conductors there will always be a corre- 
sponding one on the other, between which, if they are 
electrically connected, no current will pass. 

Let the Wheatstone bridge be represented by a dia- 
mond, Figure 154, with opposite points connected. Let 

329 



330 ELEMENTARY ELECTRICITY 

the four arms of the bridge be designated by a, b, c and 
d. If no current flows through the wire indicated by g, 
the proportions will hold: 

a :b : : c : d and a : c : : b : d. 

In a proportion, if three of the quantities are known, 
the fourth one can always be found by the arithmetical 
"rule of three." If, therefore, any three of the resist- 
ances are known, and if no current passes through g, 
the fourth or unknown resistance can be calculated by 
the rule of three. 

Suppose that an unknown resistance is to be deter- 
mined. It is placed in the bridge connection, at d it may 
be; theoretically, the place is indifferent. The current 
goes through it, and it must constitute the entire resist- 
ance of the arm d. Known resistances are put in for 
a and b. Suppose they are a = ioo ohms and b = 5 
ohms. Then one resistance after another is tried at c 
until no current passes through g. Suppose that this 
was 57 ohms. We then have the proportions : 

100 : 5 : : 57 : x or 100": 57 : : 5 : x 

from either of which we find that 
x = 2.85 ohms. 

This is the law of the Wheatstone bridge. The ap- 
paratus is one of the most used in electrical work. To 
ascertain when no current passes through g, a sensitive 
galvanoscope may be used. It need not be a galvanom- 
eter — that is to say, it need not be a measurer of cur- 
rent; it is enough if it shows the presence of a current. 
It must be sensitive, as the slightest current must be 
shown by it if it exists. 



. 



ELECTRICAL MEASURING INSTRUMENTS 



331 



Figure 155 gives a perspective view of a simple bridge 
to demonstrate the principle. 




Fig. 155. Simple Wheatstone Bridge. 




Fig. 156. Meter Bridge. 



The Meter Bridge has been used for the most delicate 
researches. The cut, Figure 156, shows the connections. 
The characteristic pert from which it takes its name is 



332 



ELEMENTARY ELECTRICITY 



the wire in this instrument stretched three times along 
its front. This wire represents two of the arms of the 
bridge. A sliding piece K moves along it, and by de- 
pressing a key connects the conductor from the galvan- 



A/VWWWWWV 



w 



M' 




Fig. 157. Principle of the Potentiometer. 




Fig. 158. Potentiometer Connections. 



ometer to the wire. This point represents the top or 
bottom of the diamond. The position of the point read 
off on the scale gives the ratio of resistance of the two 
sides represented by the stretched wire. By using one 



ELECTRICAL MEASURING INSTRUMENTS 



333 



or the other of the three leads of the stretched wire, or 
by using two or three of them simultaneously, all sorts 
of proportions between the parts. to right and left of R 
can be brought about. The known and unknown resist- 
ance represent the other legs of the diamond, and the 
point where the other conductor from the galvanoscope 
is connected is the end of the diamond. The small figure 
shows the contact piece which is moved along the wire. 



W\AV\AAAWVW 




Fig. 159. Potentiometer Connections. 

The Potentiometer is an apparatus for measurement 
of resistances, current strength and potential differences. 
It has acquired in late years most extensive application. 
Modern electric measurement practice tends, or should 
tend in the direction of null methods. The potentiom- 
eter uses one of these. A reflecting galvanometer may 
be and generally is used with the potentiometer. Its 
function is simply as a galvanoscope, just as in the 
Wheatstone bridge method. When it shows no poten- 
tial difference, the reading of the resistance coils gives 
the result of the experiment. 



334 



ELEMENTARY ELECTRICITY 



Principle of the Potentiometer. — In Figure 157 W is a 
battery giving a constant current, R is an adjustable re- 
sistance, A B is a resistance divided into 150,000 parts, 
and by movable contacts M M' different lengths of it may 
be thrown into parallel with the circuit containing the 



-O e.m.f. o 



OA/WWWWVWVWW\AW^ 




Fig. 160. High-Voltage Connections for Potentiometer. 



galvanometer G and at £ a battery not shown in place, 
because various cells are used there, E indicating the 
binding posts for connecting them. For general require- 
ments the drop between M and M' must be at least 1.5 
volts under the action of the main battery W, which is 
not a standard one. 



LESSON 21. 
Electrical Work, Power and Efficiency. 

FORCE. — Force is defined as that which produces 
motion, or a change of motion; thus force must always 
be applied to any body to cause it to move. To increase, 
decrease, or stop this motion, that is to change it, force 
must again be applied. For example, to start a loaded 
wheel-barrow force must be applied, either by pushing 
or pulling it, but when it is set in motion less force will 
be required to keep it in motion ; to cause a change in 
motion, that is to increase or decrease the speed, extra 
force must be applied. Force does not always produce 
motion, but only tends to produce it, as when a man tries 
to push a laden freight car he applies all his muscular 
force, but no motion results. 

DIFFERENT KINDS OF FORCE.— There is the 
force of gravitation, which causes all bodies free to move 
to fall from a higher to a lower level. The force exerted 
by a man riding a bicycle or a horse drawing a carriage 
are examples of muscular force. An engine draws a 
train of cars by reason of the mechanical force applied, 
which is due to the expansion of the steam in the steam 
cylinder. A mixture of air and illuminating gas in a 
room is ignited and the explosion wrecks the room; the 
action is due to the chemical force exerted. The force 
which produces or tends to produce a flow of electricity 
js electromotive force. The rate at which a train moves 

335 



336 ELEMENTARY ELECTRICITY 

depends upon the force exerted by the engine, so also, 
the rate of flow of electricity depends upon the amount 
of electromotive force applied. 

MASS AND WEIGHT.— The mass of a body is the 
quantity of matter in it; the weight of a body is due to 
the force of gravity acting upon this matter. Since the 
force of gravity diminishes as we ascend from the earth's 
surface, the attraction for a mass of matter will diminish, 
or it will weigh less on the top of a high mountain than 
at the sea level ; the mass of matter, however, would be 
the same in each case. Weight is not, therefore, the 
same thing as mass, but we can conveniently measure a 
body by its weight. 

WORK. — Work is done when force overcomes a re- 
sistance, or, work is force acting through space 
(W=FXS). 

Work=ForceX Distance, 

or Work=Pounds X Feet=Foot-pounds. 

Work is not always done when a force acts ; for in- 
stance, a man pushes with all his force against a brick 
wall; he is exerting force, but doing no work because 
no motion results, nor is any resistance overcome. If a 
weight be lifted, work is done directly in proportion to 
the weight and to the distance through which it was 
moved. Thus, the work done in lifting 4 pounds to the 
height of 3 feet is equivalent to 12 foot-pounds of work. 
Exactly the same work is performed when two pounds 
are lifted 6 feet ; or 6 pounds raised 2 feet ; or 12 pounds 
raised 1 foot. Work does not always consist in raising 
weights ; the steam engine does vvork by hauling a train, 
due to the expansive force of steam acting upon the pis- 
ton; an explosion of powder in a cannon causes an iron 



electrical Work — power" 337 

ball to traverse a certain distance. The magnetic action 
in a dynamo sets up a force which causes a current to 
flow through an electric motor and the motor drives a 
car weighing so many pounds a certain number of feet 
every minute, hence the total foot-pounds of work are 
performed electrically. The work in each case is meas- 
ured in foot-pounds. Whether work be done mechani- 
cally, chemically, thermally** Or electrically, it can be 
expressed in foot-pounds. The total amount of work 
done is independent of time, that is, the same work may 
be performed in one hour or one year. When different 
amounts of work performed in different times are to be 
compared, then reference is made to the time, or rate of 
working, or the power. 

POWER. — Power is the rate at which work is done, 
and is independent of the amount of work to be done. 

• ., - , . x Work Foot-pounds 
Power (rate of working) =~^—=- Time = 

Foot-pounds per unit of time. 

For example, it requires four hours for a particular 
engine to draw a train from one station to another, while 
another engine may draw the same train the same dis- 
tance in two hours. One engine is thus twice as power- 
ful as the other, because it can do the same work in half 
the time. When the train has reached its destination it 
would have represented the same amount of work done, 
no matter whether it had traveled at one mile per minute 
or one mile per hour, leaving, of course, friction and air 
resistance out of account. 

♦By heat. 



338 ELEMENTARY ELECTRICITY 

Power is estimated according to the amount of work 
done in a given period of time. As mechanical work is 
measured in foot-pounds, mechanical power would thus 
be so many foot-pounds per minute, or per second. The 
mechanical unit of power is the horse power. 
One Mechanical Horse Power=33coo ft. lbs. per Minute 

33000 _ r . 11 c a 

or = 55° ft- lbs. per Second. 

If a body weighing 33000 pounds be raised one foot 
every minute then we have a rate of working equal to 
one horse power; or if 16500 pounds be raised two feet 
per minute, the rate of working is the same, one horse 
power. If the work were continued at the same rate for 
one hour, we would have a larger unit of work, or the 
horse-power-hour. When we say that an engine is de- 
veloping 40 horse power we mean that it is performing 
550X40=22000 foot-pounds of work every second. 

DIFFERENCE BETWEEN ENERGY, FORCE, 
WORK AND POWER.— It is important that the stu- 
dent should thoroughly understand the meaning of the 
above terms. Energy is the capacity to do work. Force 
is one of the factors of work and has to be exerted 
through a distance to do work, the work being reckoned 
as the product of the force and the distance through 
which it has been applied. Work is done when energy is 
expended or when force overcomes a resistance. Power 
is the rate of working. 

ELECTRICAL WORK.— Work is force acting 
through space, or energy expended, therefore, resistance 
is overcome when work is performed. Force may exist 
without work being performed, as when you push against 



ELECTRICAL WORK — POWER 339 

a table and do not move it, no work is done, yet the 
force exists. An electrical force exists between the two 
terminals of a battery, tending to send a current of elec- 
tricity from one to the other through the air. The force 
is not sufficient to overcome the resistance of the air, 
therefore no current flows and the battery is not doing 
any work; the same is true with a dynamo when run- 
ning on open circuit. When a wire is connected across 
the battery terminals, the force overcomes the resistance 
of the wire and electricity is moved along, around or 
through the wire, which becomes heated. The electrical 
work, or energy expended, in this case, is represented by 
the amount of heat generated. With a small lamp con- 
nected to the battery, the work is represented by the 
heat and light given by the lamp as well as the heat 
given to the remainder of the circuit. The total work 
performed is the product of the force, the current, and 
the time that the current is maintained or 

Electrical Work=Volts X Amperes X Time. 

But the engineer is not interested much in work — the 
element of time is of great importance to him, so he 
always figures power used. 

ELECTRICAL POWER.— Power is the rate at which 
energy is expended, and is independent of the total work 
to be accomplished. The rate of working, or the power, 
is found by dividing the total work by the time required 
to perform it. 

Electrical Work 



Electrical Power 



Time. 



The unit of electrical power is a unit of work per- 
formed in a unit of time, and is called a Watt. 



340 ELEMENTARY ELECTRICITY 

Power=YoltsXAmperes=EC. 

Problem I. A current of 2000 amperes flows at d 
pressure of 600 volts. What power is used? 

Watts=E C=6oo X 2000= 1 200006; 

To avoid the use of large numbers the Kilowatt is used; 
It is 1000 watts. 

The answer to Problem 1 is therefore 1200 K. W. 
(Kilowatts abbreviated K. W.). 

Problem 2. How many K. W. will an alternator pro- 
ducing- 1 1000 volts and 2~2 amperes give? 

W=EC=i 1000X272 

=2992000 watts 
=3000 K. W. (nearly). 

A watt is a small unit, for it takes 746 watts of elec- 
trical power to exert the same power as one horse power 
of steam power. 

Hence 746 watts=i horse power (H. P.) 

and Y\ K. W.=i H. P. (approx.) 

1 K. W.=i^ H. P. (approx.) 

A rough rule for figuring is : 

To change from K. W. to H. P. add on y$ of the 
number, from H. P. to K. W., subtract Y± of itself from 
the number. 

A very convenient formula for power is obtained in 
this way: 

W=EC, but C=| 
hence E=CR, so W=CRX 0=C?R 



electrical Work— power 34i 

This means the watts power used up in any resistance 
is found by the formula W=C 2 R. The square of the 
current multiplied by the resistance. 

This is all wasted power and is often referred to as 
the "C square R loss." 

When a current goes through a motor it produces 
some mechanical power, but when flowing through a 
wire it produces nothing but magnetism and heat. This 
is often referred to by saying a current produces C 2 R 
heat, meaning that the current produces OR watts which 
turn to heat. 



EFFICIENCY. 

When we bring 223.8 K. W. of energy to a motor and 
turn out at its pulley 240 H. P., it is because some of 
the energy has been lost in the transformation from 
electrical to mechanical power. 

223.8 K. W .=223800 watts. 
223800-^746=300 H. P. 

Efficiency== S L 

Both being expressed in same units, 

a • 3QO 

emciency= =0.0. 

J 240 

Efficiencies are usually multiplied by 100 and then 
called per cent. Efficiency of 0.8 would be called 80% 
efficiency. 



342 



ELEMENTARY ELECTRICITY 



CIRCUIT BREAKERS. 

In its simplest form a circuit breaker is merely a 
switch so designed as to be capable of frequently open- 
ing the circuit carrying its full current without any 
damage to itself. 







Fig. 161. Automatic Circuit Breaker with Low Voltage Release, 
Tell-tale Switch and Magnetic Blow-out. 



With the large currents handled in railroad work 
it has become necessary to define a switch as "a piece 
of apparatus to close circuits." 



ELECTRICAL WORK — POWER 343 

Apparatus to open circuits* are now called circuit 
breakers, trip switches, oil switches or some other spe- 
cial name. 

In order to have a good contact for carrying current 
where breaker is closed, this contact is of copper. 

As shown in Fig. 161, the contact is composed of 
two large copper blocks against which press the ends 
of a curved copper brush. This brush is made of nu- 
merous thin sheets of copper, pressed together so as to 
form an almost solid block of metal. They are held 
together tightly in the middle, and the ends left free. 

When the brush is pressed upwards against the 
blocks it makes a peculiar scratching or rubbing contact 
in which each separate leaf of the brush makes its own 
contact, and holds it with a firm pressure owing to the 
springiness of the copper. 

The scratching or rubbing contact ensures the re- 
moval of all dirt or oxidized copper from the block and 
the individual action of each leaf makes a good con- 
tact, utilizing the total surface of the brush. 

While such a contact is an excellent carrier of cur- 
rent, it is the worst possible breaker of a current, for 
the separate leaves would melt on the edges and fuse 
together. 

The breaker is arranged so that a second contact of 
a carbon plug in a carbon socket always closes before 
and opens after the main copper contacts. 

In Fig. 161 this secondary carbon contact is on the 



♦It will be understood that after breaking a current by a cir- 
cuit breaker the mere opening of a knife switch cannot be called 
opening a circuit, because after the breaker opens there is no 
electrical circuit. 



344 



ELEMENTARY ELECTRICITY 



end of the rod which passes up into the hollow formed 
by the nameplate. 

In Fig. 162 the carbon plug is K and the sides of the 
socket are marked G. The main contact blocks are the 
squares and the copper brush is marked H. 

The toggle which closes and opens the breaker is 
marked F. 




Fig. 162. Diagram of Fig. 161. 



In Figs. 163 and 164 the carbon contacts are seen 
at the top in the shape of carbon blocks. In Fig. 163 
the main contact is plainly shown below, while in Fig. 
164 the main contact is concealed by a metal housing. 

The mechanical connection between the main (cop- 
per) contacts and the secondary (carbon) contacts has 
enough lost motion that the main contact is well opened 
before the secondary opens. 

What has been described constitutes a circuit breaker, 
but a circuit breaker is always an automatic device. An 
oil switch may or may not operate automatically, but 
when we say circuit breaker we always mean an auto- 
matic one. 



ELECTRICAL WORK — POWER 



345 



The circuit breaker is set by hand against a spring 
and held shut, closed, or set (the three words meaning 
the same thing) by a latch or hooked catch. There is 
a rod one end of which is arranged to unfasten or trip 




Fig. 163. Large Capacity Breaker. Carbon Break Type 2000 to 
10000 Amperes. 



the catch, the other end is fastened to an iron armature 
or core. 

All the current in the particular circuit which the 
breaker is in passes through a solenoid which exerts an 
attraction on the core or armature. 



346 



ELEMENTARY ELECTRICITY 



Suppose the normal current on the line to be 700 
amperes, then the circuit breaker would have contacts 
of sufficient area and a solenoid of such a size that 700 




Fig. 164. Carbon Break Circuit Breaker. 150 to 800 Ampere Capacity. 



amperes could pass through the breaker 24 hours a day 
without overheating any part. 



ELECTRICAL WORK — POWER 



347 



The weight of rod and core of armature would be 
sufficient so that the lifting power of the solenoid was 
not great enough to lift them, but that iooo amperes 
through the solenoid will lift the rod and release the 
catch. The spring then opens the breaker. When a 
circuit is to be opened the attendants push up the rod 
and thus open the breaker. 




Fig. 165. Magnetic Blow-out Breaker. Small Capacity. 



When this is done it is called tripping the breaker, 
when the breaker is tripped by the action of the current 
it is customary to say the breaker has "blown." 

Whether a breaker is tripped or blown, it makes a 
noise like a pistol shot. The larger the current blown, 
the greater the noise. 

The tripping solenoid is- shown plainly in Fig. 165. 



348 ELEMENTARY ELECTRICITY 

If large currents are handled by the breaker a sol- 
enoid of one-half a turn may create enough power to 
trip the breaker. 

In Figs. 163 and 164 this is the case, and the tripping 
device is not in evidence. 

If the rod and its attachments were heavier it might 
take 1200 amperes to trip the breaker; if the core or 
armature were moved nearer or further from the sol- 
enoid, a smaller or larger current would trip the 
breaker; if a more or less strong spring were attached 
the current required to blow the breaker would vary 
with the tension of the spring. 

These devices are used to set the current at which 
the breaker will operate. 

Breakers are made to have a capacity for certain cur- 
rents continuously passing through them without over 
heating. This is called their Continuous Capacity. 

The carbon contacts are then designed to break a 
Current of from 50% to 100% in excess of this current 
for several hundred times without needing renewal of 
parts. This is called the maximum capacity. 

Sometimes the maximum capacity is two or three 
times the continuous capacity. Such breakers must be 
very heavily and strongly built, and are much higher 
in price. 

The lowest current at which the breaker can be set 
to operate is called its minimum calibration. 

In Fig. 163 at the left side is a rod screwed in an 
arm. The lower end of this rod has a flange which 
holds an armature from falling down, but does not 
prevent its rising. 

The upper end of the rod moves past a scale marked 
in amperes. Turning this rod till its head is opposite 



ELECTRICAL WORK POWER 349 

a certain number draws the armature into such a posi- 
tion that the indicated number of amperes will pull the 
armature up and release the catch or trigger, which can 
be plainly seen holding the arm which operates the 
toggles in place. 

It is evident that the scale reads from the top down, 
for then setting screw at smallest number brings the 
armature nearest the solenoid. Fig. 165 shows the arma- 
ture above the solenoid with a spring to change the 
pull required to draw it down and release the trigger. 

In this breaker the main contacts are between the 
handle and the solenoid, while the secondary contacts 
are up top behind the name plate. 

In Fig. 166 is shown a breaker with a core in the 
solenoid. 

The main current circulates around the solenoidal 
coil "B" and tends to draw into the solenoid the mova- 
ble plunger "C." The initial position of this plunger 
in the solenoid is determined by the adjusting screw 
"M." When the current is sufficient to overcome the 
weight of the plunger it is drawn into the coil with 
constantly increasing velocity, due to intensified mag- 
netic action, as the polar distances or air space is de- 
creased. When nearing the upward limit of its travel, 
having acquired a high momentum, it impinges upon 
the trigger "N" through the medium of the push pin 
"E." The immediate result of this is the release of 
the switch arm by the displacement of the retaining 
catch "F." The upper projection "H" of the trigger 
"N" is thrust against the striker plate "K," thereby 
utilizing the energy of the current to start the movement 
of the switch arm. This movement is intensified and 
sustained beyond the point of final rupture between the 



350 ELEMENTARY ELECTRICITY 

switch contacts by the thrust of the spring- "O," which 
is released from compression by the initial action of the 
trigger. Thus the contact arm is thrown away from 
the contact terminal, and the circuit is opened. 

As the screw "M" is turned up and locked by "T M 
it prevents the core "C" from falling away from the 
solenoid. The higher "C" the lower the current at 
which breaker blows. 

The fact that the current is broken between the 
carbon blocks tends to suppress the arc formed, and in 
the breakers shown in Figs. 163, 164, and 166 this is 
alone relied on to kill the arc. 

It must be remembered that although these breakers 
have two contact blocks at the main copper contact, 
yet only one wire of the circuit is attached to the 
breaker. The current enters at one block, goes over the 
copper brush to the other and out to the line. The 
carbon contacts are a shunt. Circuit breakers are 
adapted to different voltages by the excellence of the 
insulation and by the length of the openings between 
contact pieces. 

The breakers shown are all suitable for D C and A C 
circuits of 100 to 800 volts. 

The breakers shown in Figs. 161 and 165 have a 
magnetic blowout; that is, a solenoid is situated at the 
carbon contact, which actually blows the arc out the 
same as in the Thomson lightning arrester. 

The breaker in Fig. 161 has two attachments which 
are of great service. These are the Low Voltage Re- 
lease and the Tell Tale. 

The way these act is best explained in connection with 
Fig. 167. 

The Low Voltage Release is a coil of low resistance 






ELECTRICAL WORK — POWER 



351 




Fig. 166. Cross Section of Circuit Breaker. 



352 



ELEMENTARY ELECTRICITY. 



holding by its armature the trigger of the breaker. A 
resistance is placed in series with this coil and both 
together are placed as a shunt across the line. In the 




-i- Grouno 

Fig. 167. Electrical Connections of Fig. 161. 



diagram one side of the release coil is connected to 
ground, which is negative, and the other side through 
the series resistance to the positive brush of rotary 
converter. 



ELECTRICAL WORK — POWER 253 

As long as the voltage is normal this coil is a strong 
enough magnet to hold the breaker trigger set, although 
at any time the main solenoid can release the trigger 
independently of the low voltage coil. 

If the voltage falls to half the normal pressure the coil 
becomes such a weak magnet that the trigger is re- 
leased and breaker opens. 

Wires are often run from the terminals of the low- 
voltage coil to push button in different parts of the 
station. 

Pushing the button then forms a short circuit across 
the low voltage coil and robs it of its current. It ceases 
to be a magnet and the trigger is released. The breaker 
can thus be opened from several distant points. 

To prevent rotary converters from racing at great 
speed, when power on A C side is suddenly thrown off, 
there is a speed limit device on the rotary which con- 
sists of a ring in which a fly-ball governor rotates. The 
ring is connected to one side of the low voltage coil and 
the fly-balls through the negative wiring, to the other 
side. When the rotary goes too fast the fly-balls open 
out and touch the ring, completing a short circuit across 
the low voltage coil and thereby releasing the trigger. 

The Tell Tale is merely a mechanically operated 
switch, which the opening of the breaker closes. The 
tell tale may close the circuit' of an electric bell and call 
the attendant's attention. 

It usually rings a bell and lights a lamp. It may 
even trip a second breaker, if it is desired to always 
have the two "go out"* at the same time. 

For higher voltages, 6600 volts and upwards, break- 



*Another expression for "blowing." 



354 



ELEMENTARY ELECTRICITY 




Lifting Tabl» 



Lifting Cam 



Fig. 168. Oil Circuit Breaker. 



ELECTRICAL WORK — ROWER 355 

ers of type shown in Fig. 168 are used. A magnet 
when current is too large operates the valve which ad- 
mits air to the cylinder. The cylinder piston through a 
wooden rod operates the main contact first and then the 
secondary contacts which are enclosed in oil rilled 
cylinders. 



LESSON 22. 
Direct and Alternating Current. 

We have seen that when direct current, as it is called, 
passes through an electro-plating bath, there is a trans- 
fer of chemicals in both directions. The metals go from 
the anode to the cathode, i. e. from + to — negative 
wire. The non-metals such as sulphur, chlorine, etc., 
are transferred from the cathode to the anode. 

It seems as if direct current flowed in both directions 
around the circuit, but since the metals are of the most 
importance to us we speak and often think of direct cur- 
rent as flowing only in one direction. 

The test for direct current is that it will electro-plate. 

Ohms Law in its simple forms as mentioned in Les- 
son 19, applies absolutely to direct current. 

It is a fact that for the first fraction of a second after 
a switch is closed the current is growing to the value 
it ought to have as given by Ohms law. 

This growth is very rapid when the circuit contains 
no coils or magnets. These retard the rise of the cur- 
rent. 

The current in the field circuit of a dynamo may take 
3 seconds to attain its full value, but once there its value 
is given by Ohms Law. 

Alternating Current. 

An alternating current will not electro-plate, for the 

metal-plating part of the current reverses direction many 

times a second. 

356 



DIRECT AND ALTERNATING CURRENT 357 

An alternating current will not deflect a magnetic 
needle because the deflecting impulse reverses its direc- 
tion continually. 

Alternating current can excite a magnet causing 
a core to be sucked into a solenoid or an armature to 
be attracted, because induced magnetism in core changes 
with the polarity of magnet itself. 

Alternating current measuring instruments do not 
contain magnetic needles, but are supplied with two 
coils. The magnetic action between these two coils is 
just as strong as if direct current were used, because 
the A. C. reversing in each coil at the same time keep9 
the relative polarities the same. 

Other A. C. instruments like the Thomson Ammetef 
Fig. 157, exert a magnetic action on a vane. This vane 
is not itself a magnet. 

Since A. C. does not electro-plate the original defini- 
tion of an ampere is of no use here, but since the heating 
done by electricity is the same for equal quantities 
whether A. C. or D. C. we define thus: 

An ampere of A. C. is that current which produces as 
much heat per second as one ampere of D. C. 

For example : A car heater raises the temperature of 
the car from 6o° Fah. to 70 in half an hour. The 
ammeter read 20 amperes D. C. If A. C. were supplied 
and raised the temperature from 70 to 8o° in the next 
half hour we would say that 20 amperes A. C. were flow- 
ing. 

Ohms Law applies to A. C. when written thus : 

r Impedance 

- L ~~' Volts 



358 ELEMENTARY ELECTRICITY 

The impedance is larger than the resistance of the cir- 
cuit, it being the result of the resistance and the react- 
ance. 

Resistance is the opposition offered by a conductor to 
any or all current passing. 

Reactance is the extra opposition offered by a con- 
ductor to a current which is changing in vaJue. It de- 
pends on the rapidity of this change and on the number 
of coils of wire in the circuit. It is measured in ohms 
and is calculated by the formula: 

Reactance=6.28XfXL 
where f=frequency 

L^coefficient of Self-induction. 

The frequency, as explained before, is the number of 
times the current reverses per second. It runs from 
25 in power and railroad work up to 133 in electric 
lighting. It is best determined by taking the speed of 
the alternator in the power iiouse and counting the num- 
ber of pairs of poles that alternator has. 

Calculating the number of pairs of poles passed a 
second gives the frequency. 

Problem: An alternator makes 75 revolutions per 
minute and has 40 poles; what is the frequency of the 
current delivered? 

40 poles=20 pairs poles 
75 r. p. m.=i.25 r. p. second 
20X1.25=25 pairs poles per second. 
25=frequency. 



DIRECT ANt) ALTERNATING CURRENT 359 



Self Induction. 

When the current changes strength in a long straight 
wire the resistance of the wire does not resist the change, 
for the resistance offers the same opposition to any or all 
currents ; it is the magnetic field around the wire which 
causes the reactance of the wire. 

Every current has its definite magnetic field in the air 
around the wire and when current is increased the mag- 
netic field has to increase to its new value. When the 
current alternates the rapid dying away and regrowth 
of the magnetism consumes some of the power so that 
for the same E. M. F. the current is less when it is A. C. 
than when D. C. 

The greater the frequency the greater is the reactance 
of the circuit because the more frequent the rise and fall 
of the magnetic field about the wire. 

Perhaps it would be truer to facts to say that the rise 
and fall of the magnetism causes it to cut through the 
wire and produce an E. M. F. opposite to that produced 
by the alternator which reduces the net E. M. F. im- 
pressed on line. 

When coils of wire are present this action is much 
stronger because the magnetism cuts the circuit oftener 
on each rise and fall. 

The action of the coils is called Self-induction and the 
number representing the action is called henrys, named 
after Henry, a noted electrician. 

It is well to remember that the reactance of the cir- 
cuit is caused by the frequency of the A. C. and the 
self-induction of the circuit. 



360 ELEMEftf ARY ELECfRiCITY 

Prohleni: Suppose a circuit of 0.12 henrys self-induc- 
tion and 184 ohms resistance conducts A. C. at a fre- 
quency of 60. What will be the current flowing with 
1 1000 volts 

Reactance=6.28 X f X L 

=6.28X60X0.12 
=45 ohms 
Resistance=84 ohms 

Impedance=Square root of the sum of squares of re- 
sistance and reactance. 

Resistance squared=7056 ohms 
Reactance squared=2025 
Sum of squares=cp8i 
Square root=95.3 
Impedance=95.3 ohms. 

~ Volts 1 1000 

0=t 1 = = II 5-4 amperes, 

Impedance 95.3 ^ r 

Capacity. 

The presence of condenser action or capacity in a A. 
C. circuit is a help because all circuits have some in- 
ductance (self-induction plus the effect of any circuits 
near the one in question), and the capacity tends to 
neutralize this, bringing the value of the impedance 
nearer to the resistance. 

It is even possible to artificially make the capacity so 
great that the impedance is practically equal to the re- 
sistance. 

Such a circuit would conduct A. C. or D. C. equally 
well. 



DIRECT AND ALTERNATING CURRENT 361 

Lagging, Leading Currents. 

If a shunt is put in a circuit and an ammeter con- 
nected to it, while a voltmeter is connected to the same 
part of the circuit, if* readings could be taken of the 
values of current and voltage at any instant of time 
we would find that the highest value of voltage and cur- 
rent did not appear at the same time. 

We would see that the value of the current as given 
by C=Volts-f- Impedance, occurs a fraction of a second 
after the voltage causing the current has passed. We 
say the current lags behind the voltage. If we add 
more inductance to the circuit the current will lag 
more and more until it seems to the observer at an 
oscilligraph that the current can hardly belong to the 
voltage he is watching, because the largest current is 
flowing when the voltage is almost down to zero and 
positive current is flowing when the voltage has re- 
versed and is negative. 

If capacity is now connected to the circuit the current 
will move up to a position nearer where it naturally 
should be until with sufficient capacity it will follow 
exactly the changes in the voltage. That is the highest 
current and voltage will occur at exact instant. 

Add more capacity and remove all inductance possible 
by taking out of circuit all coils or machinery contain- 
ing coils, especially if they have iron cores. 

The current will actually lead the voltage. That is, 
the highest current will flow a fraction of a second be- 
fore the highest voltage occurs. 



♦An instrument called an oscilligraph can make these instan- 
taneous readings. 



362 ELEMENTARY ELECTRICITY 



Power Factor, Wattless Current. 

The power in this circuit while all these changes have 
been going on has varied greatly. When the current 
and voltage were together the power was the greatest, 
when the current either led or lagged the power was 
less. This is because the power is determined by mul- 
tiplying the Current and voltage which occur at the 
same time. It makes no difference whether that voltage 
belongs to that current or not. It is the product of the 
things which are happening at the same time which 
gives the power. 

If an A. C. ammeter and voltmeter are put in cir- 
cuit each instrument reads the average current or volt- 
age irrespective of the time at which these currents or 
voltages occurred. Multiply these two readings and you 
get the power produced by the current and the voltage 
which belongs to it. If you should now place a watt- 
meter in the circuit it will measure the actual power, 
i. e. the average of current multiplied by the voltage 
which occurs at the same instant. 

This wattmeter reading is the actual power. The 
ammeter reading multiplied by voltmeter reading gives 
the apparent power. 

The decimal number by which the apparent power 
must be multiplied to get true power is called the Power 
Factor. 

If a current of 250 amperes is flowing at a pressure 
of 1000 volts we have an apparent power of 250000 
watt or 250 K. W. If the true power as read by the 
wattmeter is 200 K. W. the power factor must be o.& 
for 256X0.8=200. 



DIRECT AXD ALTERNATING CURRENT 363 



^ True power 

Power Factors- 



Apparent power 

_ Wattmeter reading 
Volts X amperes 

By saying that the pressure measurement is correct 
we throw the blame on the current and say that all the 
current which flows, say 250 amperes at iooo volts, 
is measured by the ammeter, while only the current 
which works or produces watts is taken into considera- 
tion by the wattmeter. According to this idea the 
wattmeter would only notice 250X0.8=200 amperes. 
The other 50 amperes are called the wattless or idle 
current. 

It is customary to speak of A. C. waves. This is quite 
natural for the rise and fall of voltage and current in an 
A. C. circuit reminds one of the waves in a long string 
which is shaken to and fro at one end while being held 
fast at the other. 



TRANSFORMING DIRECT OR ALTERNATING 
CURRENT. 

Perhaps one of the greatest objections to the use of di- 
rect current is the inability to change its voltage without 
the use of moving machinery. 

There is but one way to transform direct current and 
that is by a motor and generator. 

This motor-generator set usually consists of a direct 
current motor driven by current at the pressure of the 
incoming line. This motor drives a direct current gener- 
ator which furnishes current at the desired pressure. 

By altering the strength of the field of the generator 
we regulate the outgoing pressure to suit the require- 
ments. 

The motor and generator are built on the same shaft 
and set on a long cast iron base, making them mechani- 
cally one machine. 

When the incoming and outgoing voltages can have 
the same ratio to each other always, a cheaper form of 
machine can be used called a Dynamotor. 

This is a direct current motor running on the incoming 
voltages. On the same armature core is a separate wind- 
ing connected to its own commutator at the other end of 
armature. The one set of field magnets serves for the 
motor winding and the generator or dynamo winding. 



364 



ROTARY CONVERTERS. 

As the sub-station is the link between the transmission 
line and the trolley wire, so in like manner the rotary 
converter is a connecting link between alternating and 
direct current systems — it combines in a single machine 




_ 



Fig. 169. Typical Rotary Converter Switchboard. 



the functions performed by the two machines described 
above. Thus in one sense it may be regarded as an alter- 
nating-current synchronous motor, driving a direct-cur- 
rent generator, or if the machine be inverted, it may be 
considered a direct-current motor driving an alternating 
current generator. 

365 



366 ELEMENTARY ELECTRICITY 

As direct current cannot readily be generated at or 
transformed to a high voltage, which economical distribu- 
tion dictates, alternating current is almost invariably used 
for all except very small electrical. power transmissions. 
Wherever direct current is used, as in direct current rail- 
way lines, the alternating current must be transformed 
into direct current. While, of course, this can be accom- 
plished by means of a motor-generator set, consisting of 
an alternating current motor connected to a direct cur- 
rent generator, the higher efficiency and lower cost of the 
rotary converter accounts for the almost universal prac- 
tice of using it in preference to the motor-generator on 
low frequencies. 

Rotary converters have many of the features which dis- 
tinguish the most modern direct-current machines ; the 
only material difference being the addition of collector 
rings connected to certain points of the armature wind- 
ing. The number of such connections depend on the 
number of poles and phases. 

The machine is built for single-phase, two-phase, three- 
phase or six-phase circuits, although single-phase and six- 
phase converters are seldom, desired. A two-phase con- 
verter is provided with four collecting rings and a three- 
phase converter is provided with three collecting rings. 
As it is usually found expedient to transmit the alternat- 
ing current at high pressure, transformers must be em- 
ployed for lowering the potential to secure the proper di- 
rect voltage. Where the converter is operated from di- 
rect to alternating current, transformers are usually em- 
ployed to raise the voltage for transmission. 

A rotary converter may be separately excited, but it is 
usually shunt wound or compound wound, depending 
upon the nature of the service. When the load is vari- 



ROTARY CONVERTERS 



367 



able, as in railway service, the machine is compound 
wound, which tends to maintain the direct current volt- 
age constant, by compensating for the drop in the supply 
circuit as the load comes on. The ratio between the al- 
ternating and direct current voltages of a rotary con- 
verter depends upon the number of phases, upon the wave 
form of its alternating current, upon the lead given to the 




Fig. 170. Allis-Chalmers 25 Cycle Rotary Converter (Commutator End). 



direct current brushes and to a slight extent upon the 
field excitation. In any given converter, therefore, the 
direct current voltage depends practically upon the volt- 
age of the alternating current applied. To a smaller ex- 
tent it depends upon the armature drop, which diminishes 
the voltage ratio in slight proportion with the load when 
running a. c. to d. c. and increases the ratio when running 



368 ELEMENTARY ELECTRICITY 

d. c. to a. c. This will be seen readily by referring to a 
saturation curve. With a sine wave the ratios of conver- 
sion are approximately as follows : 



Single Phase 


Two Phase 


rhree Phase 


Six Phase 


' 71 


•71 


.61 


.71 or .61 




Fig. 171. Allis-Chalmers 25 'Cycle Rotary Coaverter (Collector End). 

Thus, if the direct current voltage be 550 volts, the 
alternating current, if two phase, will be .71 x 550 or 390 
volts, and if three phase, it will be .61 x 550 or 335 volts. 
The ratio of conversion in the six-phase converter is .71 
with the star connection or .61 with the double-delta con- 
nection. The variation from these figures with wave 
forms in commercial use are taken into account in the. 



ROTARY CONVERTERS 369 

ratios of the transformers used in eonnection with the 
rotary converters: 

The rotary converter built by the Westinghouse Com- 
pany presents in its frame the same mechanical features 
as are found in its well-known line of direct current 
machines. In the smaller sizes the frame, bed plate and 
bearing supports are cast in one piece, thus giving a rigid 
construction which permits of handling the machine as 
a unit. In machines of large size these parts are usually 
separate castings in order to limit the weight of the pieces 
to be handled. This construction also allows of the ad- 
justment of the component parts of the machine to main- 
tain the central position of the armature. 

The machine is of the multipolar type, having laminated 
steel poles cast or bolted to its iron yoke and carrying eas- 
ily removable field coils. If the windings are compound, 
the series and shunt coils are insulated separately. The 
armature is of the slotted drum type with either a two- 
circuit or multiple type of winding. The coils are ma- 
chine wound and they are of such a shape as to give a 
thoroughly ventilated winding. 

The number of poles in a rotary converter is dependent 
on the speed of the armature and the frequency, as is 
the case with all alternating current machinery. This 
particular feature accounts for the difficulty in designing 
rotary converters for high frequencies, for the maximum 
armature speed is limited by the maximum safe speed of 
the periphery of armature and commutator. With a 
given speed, however, the number of poles is proportional 
to the frequency, and, with a given maximum speed of 
armature and commutator periphery, the distance between 
adjacent poles and, therefore, adjacent brush holders, is 
inversely proportional to the frequency. With high volt- 



370 



ELEMENTARY ELECTRICITY 



age 60 cycle converters these facts necessitate high com- 
mutator speeds, short distances between poles and between 
brush holders, narrow commutator segments and a high 
voltage between adjacent segments, resulting in a ten- 
dency to flashing over between brushes at sudden over- 




Fig. 172. 



Westinghouse 1,500 K. W. Rotary Converter, Pennsylvania, 
New York and Long Island R. R. 



load. This should always be borne in mind when choosing 
the frequency of a system on which rotary converters are 
to be used, and it applies particularly to 500 and 600 volt 
railway rotary converters, which are required to with- 
stand much more severe service than is ever experienced 
on lighting systems. 



ROTARY CONVERTERS 



371 



In the erection of a rotary converter the following con- 
siderations should as far as possible be observed: 

First. It should not be located in a position where it 
would be liable to exposure to moisture, as from drip- 
ping pipes, or escaping steam. 




Fig. 173. 



Westinghouse 300 K. W. Rotary Converter, 600 D. C Volts, 
500 D. C. Amperes, Three-Phase, 60 Cycles. 



Second. It should not be exposed to dirt or dust, es- 
pecially from coal. 

Third. It should be located in as cool and well ven- 
tilated a place as possible. The temperature of the ma- 
chine depends upon the temperature of the air surround- 
ing it. 



372 ELEMENTARY ELECTRICITY 

Fourth. It should be so located as to allow easv access 
to the alternating current brushes, and also to the com- 
mutator. These are the parts requiring special attention. 

tary converters should be set on substantial founda- 
tions in order to prevent vibration when running. 

The following list of instructions refer more particu- 
larly to Westinghouse machines but much of it will apply 
to others. 

Insulation of Frame. Whether the frame should be 
insulated from the ground is a matter to be determined 
by the engineer iri charge of the plant, but rotary con^ 
verters are usually not insulated. However, the following 
remarks which apply to alternating current practice may 
not be amiss : 

Generally speaking, the strain on the insulation of the 
windings will be decreased and the danger to the attend- 
ant increased by insulating the frame. "When, however, 
it is considered advisable to insulate the frame, the foun- 
dation should be capped with a stout wooden frame bolted 
down." The bolts which hold this frame to the masonry 
should not come in contact with those which hold the 
machine to the frame nor should anv metal or electrical 
conductor ;::n the two sets of bolts. The wooden in- 
sulating frame under the machine may also be covered 
with some insulating waterproof paint or compound. 

Erection of machine. When placing the parts of a 
machine in position the following points should be ob- 
served: 

(i) Set the lower half of the field in position and 
place the armature in its bearings, having first carefully 
examined the bearings and oil wells to be sure that they 
are clean and free from dirt. Be sure that the oil rings 
are in place and in good running order. 



ROTARY CONVERTERS 



373 







s?. 



9^3 



374 ELEMENTARY ELECTRICITY 

(2) Clean the contact surfaces of both halves of the 
field and file off the burs, if any exist, to secure perfect 
magnetic joints at the division of the yoke. 

(3) Set the upper half of the field in position and 
secure it to the lower half by means of the field bolts and 
feather keys. 

1 4 ) Xote that the machine is to be perfectly level 
along the axis of the shaft, except that when an oscil- 
lator is attached the machine is placed slightlv out of 
level, as will be pointed out later. 

Armature. Never try to support any of the weight of 
an armature by the commutator or collector rings. Do 
not allow these parts to rest on any blocking, and do not 
pass a rope around them for the purpose of lifting. When 
handling the armature always support it with a rope 
slung about the shaft, and be careful not to mar or 
scratch the shaft, as any roughness would cause it to cut 
the bearings and so produce heating when the machine 
is running. 

In putting the armature in the field be careful not to 
scratch the bearings nor to bend the oil rings. 

Coils. Assembly or field coils. The field coils of the 
larger machines are shipped separately. The coils are 
held on the poles by the dampers, which should be bolted 
to the pole pieces. The coils on each of the separate 
halves of the field should be properly connected before 
the machine is set up. 

Each pole piece has a number stamped with steel sten- 
cil and also painted in red. and a red line is drawn parallel 
and near to one edge of the pole. This line and number 
correspond to similar marks inside the coil. In erecting, 
place the coils on the poles so that the marks coincide. 

If a rotary converter has been exposed to dampness it 



ROTARY CONVERTERS 375 

may be dried out by employing one of the following 
methods : 

( i ) Short-circuit the field and apply to the collecting 
rings about 10 per cent of normal alternating current 
voltage, which may usually be obtained from the lower- 
ing transformers. During this application the d. c. 
brushes must be raised and the rotary must be at a stand- 
still. The standard Westinghouse transformers are usu- 
ally provided with taps between which a low voltage may 
be obtained. 

(2) Run the rotary converter, driving it by a suitable 
motor, and short-circuit the armature on the direct cur- 
rent side with very weak field excitation. If shunt 
wound, separate excitation at very low voltage must be 
used. If the converter be compound wound, the arma- 
ture may be short-circuited through the series field coils. 
As rotary converters are usually very sensitive as series 
machines this method should be undertaken only by those 
who are thoroughly experienced, as there is danger of 
excessive current. 

(3) Dry the field coils from a source of separate ex- 
citation, with about two-thirds of the normal d. c. volt- 
age. This will also dry the armature somewhat. While 
drying out, the temperature of the accessible parts should 
be watched closely, and not be allowed to exceed 75 ° C. 

In drying out with current there is always danger of 
overheating the windings, as the inner parts may get 
injuriously 'hot because they cannot quickly dissipate the 
heat generated in them. Coils containing moisture are 
more easily injured by overheating than those which are 
already dry. Several hours, or even days, may be re- 
quired for thoroughly drying out. 



376 



ELEMENTARY ELECTRICITY 



REPAIRS TO ARMATURE COILS. 

Fig. 175 shows a section of the strap wound armature 
coils. The coils on the armature are either strap-wound 
or wire-wound. They may be arranged either in a trio 
circuit, or multiple type of winding. Care should be 
taken that the insulation be not abused, or it will not 




Fig. 175. Westinghouse Strap-Wound Rotary Converter Armature Coils. 



insulate. A blow upon a coil which forces one wire 
through the cotton insulation to the next wire will cause 
a short-circuit, and a bruise or bend in the insulating 
cell of *an armature coil may cause a ground. 



ROTARY CONVERTERS 377 

If a defect in insulation appears on an armature coil, 
or on the outside of a field coil, it can often be repaired 
by carefully raising the injured wires, and putting fresh 
around them. The coils are held in place in the arma- 
ture slots by means of wedges. In replacing a coil these 
wedges must be driven out from a sufficient number of 
slots to permit of the raising of the winding and the 
removal of the defective coil. The new coil can be placed 
in the slot, the wedges replaced and connections made as 
before. In lifting coils out of the slots care must be 
taken not to break or strain the insulation. 

Commutator. This is a most important part of the 
machine and requires careful attention. Its surface should 
be kept smooth. 

If the commutator is very rough it may be dressed up 
with a piece of sand stone, the proper quality to be de- 
termined by trial. The stone is held firmly in the hands 
and pressed against the commutator. After using the 
stone the smoothing of the commutator may be finished 
with No. oo sandpaper. If the commutator is not very 
rough the sandpaper may be used alone. The use of 
emery cloth is not permissible on account of the grit it 
leaves on the commutator surface. It is advisable to raise 
the brushes when sandpapering the commutator. 

Ordinarily the commutator only requires to be wiped 
off with a piece of canvas, but it should be kept lubri- 
cated by the use of a very small quantity of vaseline or 
oil applied with a piece of cloth. Do not use w T aste. 

See that none of the segments of the commutator is at 
all loose. 

If the commutator should get out of true it will be 
necessary to turn it down. This can be done while the 
armature is in its bearings by using a special slide rest 



378 ELEMENTARY ELECTRICITY 

and running very slowly, or the armature may be re- 
moved and the commutator turned down in a lathe. 

Flat spots, or "flats" sometimes occur in a commutator, 
usually caused by excessive wear or by a loose bar in the 
commutator. At times a bad short-circuit may produce 
a flash that will start a "flat." 

After long usage and much wear a commutator will 
sometimes get hot when carrying only normal load. This 
indicates that it is worn down about as far as it is safe 
to go and that it should be replaced by a new one. 




Fig. 176. Westinghouse Rotary Converter Armature from the Collector 
Side, 300 K. W., 550 Volts, Three-Phase. 

Trouble in commutators is sometimes caused- by the 
burning out of the mica insulation between bars. This 
may be due to several causes, the most common of which 
is oil-soaked mica or loose commutators which allow oil 
and particles of conducting material to> work in between 
the mica and the bars. 

It has rarely, if ever, been definitely traced to excessive 
voltage between bars although this is a cause commonly 
advanced, When this burning occurs it may be stopped 



ROTARY CONVERTERS 



379 



and its effect remedied by scraping out the burnt mica 
and filling the space with a solution of silicate of soda 
(water-glass) or other suitable insulator. Trouble is also 
caused by high mica. This will start sparking which 
will burn the copper, thus aggravating the trouble. It 
may be remedied if caught in time by simply cutting the 
high mica 1/64 to 1/32 of an inch below the surface of 
the commutator. This can be done by means of any suit- 
able cutter such as a hack-saw blade held between suitable 
guides. 




Fig. 177. Westinghouse Complete Rotary Converter Armature from the 
Commutator Side, 300 K. W., 550 Volts, Three-Phase. 

Collector. The collector, in comparison with the com- 
mutator, requires little attention. It should nevertheless 
be given sufficient care to insure its running true and 
without being cut by the brushes. When cutting does 
occur it is frequently caused by the stiff brass piece on 
the back of the brushes bearing on the collector. The 
collector may be trued up with sandstone if it is not too 
much out of true. The operation of the collector can 
usually be improved by means of an oscillator. 



380 ELEMENTARY ELECTRICITY 

Brushes. These are of carbon on the d. c. side and 
copper on the a. c. side. The position of the d. c. 
brushes when the machine is converting from alternating 
to direct current should be slightly in advance of the no- 
load neutral point on the commutator. 

A dry brush of carbon sliding over copper is likely to 
chatter, and therefore brushes of the sliding type are lia- 
ble to make more or less noise unless they receive proper 
attention. The noise can be avoided by keeping the com- 
mutator lubricated or by boiling the brushes in vaseline or 
dynamo oil. Boiling once in oil will last for a consid- 
erable length of time. 

The brushes on machines having a swivel type of 
holder should be adjusted as follows: 

(i) Arrange brush arms so that all are equally dis- 
tant from the commutator face. 

(2) Adjust carbons in holders so that the stops will 
allow % inch to 3/16 inch wear on the heels of the 
carbons. 

(3) Grind the carbons with sandpaper until a good fit 
on commutator is obtained. 

Carbon brushes should be equally spaced around the 
commutator. The best way to space them is to divide 
the circumference of the commutator into as many spaces 
as there are poles. This may conveniently be : done by 
placing a strip of paper around the commutator exactly 
equal in length to its circumference and afterward divid- 
ing the strip of paper according to the number of poles 
in the machine. The paper may then be placed upon the 
commutator and the brushes set upon the spaces of equal 
division. This method is more accurate than that in 
which dependence is placed upon the uniform spacing of 
the bars. 



ROTARY CONVERTERS 381 

Sparking. — Sparking at the brushes of rotary con- 
verters may occur from any one of the following causes. 

( i ) Brushes may not be on the proper point of com- 
mutation. They should always have a little forward lead 
without regard to the power factor at which the converter 
is running. 

(2) Brushes may be wedged in their holders. 

(3) Brushes may not fit the commutator surface. 

(4) Brushes may not have sufficient pressure on the 
commutator. 

(5) Brushes may be burned on the end, or covered 
with copper. 

(6) Commutator surface may be rough. The com- 
mutator surface should be of a dark glossy appearance. 

(7) A commutator bar may be loose, or may project 
above the others. 

(8) There may be a "flat" on the surface of the com- 
mutator. 

(9) The commutator may be dirty, oily, or worn out. 

(10) The machine may be overloaded. 

(11) There may be pumping or hunting. 

(12) There may be high mica. 

Bucking. Bucking is the expressive name given to the 
action of the rotary converter when arcing occurs between 
two adjacent brush holder arms, thereby short-circuiting 
the machine. Bucking is, in general, due to abnormally 
high voltage or a path of low resistance over the com- 
mutator surface or to abnormal commutation conditions. 
The poorer the commutation the more liable will the ma- 
chine be to buck whenever these abnormal operating con- 
ditions occur. Some of the particular causes for bucking 
are the following: 

(a) Rough or dirty commutator. A drop of water 



382 ELEMENTARY ELECTRICITY 

falling on the commutator has been known to cause the 
machine to buck. 

(b) Excessive voltage due to increase in a. c. voltage. 

(c) Excessive voltage due to static disturbances from 
lightning arrester short-circuits. 

(d) Excessive voltage due to static discharge from or 
through lowering transformers. When bucking is due 
to this cause, it will usually occur when switching is 
done in the high tension circuits. 

(e) Bucking may be caused by fluctuations in the 
voltage, due to the removal of a short-circuit. 

In multiple wound rotary converters, balancing rings 
or cross-connections are employed as in direct current 
generators. These rings connect together points of equal 
potential around the commutator. 

By this means the same field strength is obtained under 
each pole. 

Oscillators. The armature of a rotary converter re- 
volving with its horizontal shaft will take up normally 
a fixed position relative to its bearings, and the frame of 
the machine, and revolve without any tendency to move 
or oscillate in the direction of its length. This is detri- 
mental to its best operation, as the brushes are liable to 
wear grooves in the commutator and collector. 

The same is true of generator and motor armatures, 
although in a smaller degree, for the reason that these 
machines usually have some external force, belt, gear, or 
engine shaft, tending to produce a slight oscillation, while 
the armature of the rotary converter is free to remain in 
one position. It therefore becomes necessary to provide a 
means of producing a periodic movement of the armature 
shaft in the direction of its length, and this function is 
performed by the oscillator. 



ROTARY CONVERTERS 383 

There are two classes of oscillators, viz.: mechanical 
and magnetic. The mechanical oscillator employed by 
the Westinghouse Co. is described as follows : 

"This device is self-contained and is carried over one 
end of the shaft. The operating part consists of a steel 
plate grooved by a circular ball race in which travels a 
hardened steel ball. The steel plate is not quite parallel 
to the face of the end of the shaft. The normal position 
of the ball is at the lowest point of the circular race. The 
steel plate is backed by a spring. 

The machine is leveled so that the armature is slightly 
inclined toward the oscillator. The steel plate is then 
adjusted so that when the ball is at 'its bottom position 
it just comes into contact with the shaft-end. As the 
armature revolves the ball is carried up the race, and, ow- 
ing to the inclination of this race, compresses the spring. 
The reaction of the spring drives the shaft away. Thus 
the armature receives an impulse which moves it toward 
the other limit of its travel, and it continues to move 
until the opposing forces bring it to rest and start it back 
to its normal position, and where it again comes in con- 
tact with the ball and the operation begins over again. 

Fig. 178 shows a diagrammatic view of the Westing- 
house Magnetic Oscillator, of which the following is a 
description : 

A magnet is mounted upon one of the bearing hous- 
ings of the rotary converter in such a manner as to at- 
tract the end of the shaft. When the circuit is closed, 
the magnet draws the shaft toward it and when the cir- 
cuit opens, the armature tends to resume its normal posi- 
tion which is determined by the leveling of the converter. 
The magnet has in series with it a make and break device 
called an interrupter which is controlled by a dash-pot 



384 



ELEMENTARY ELECTRICITY 



to secure the proper frequency of action. As the dash- 
pot offers an adjustable resistance the frequency of the 
impulses are adjustable. When there are a number of 
rotary converters in the same sub-station, the magnetic 
oscillators are connected in series and controlled by a 
single interrupter. Under certain conditions the commu- 
tator and collector surfaces of machines provided with 
oscillators may be worn in irregular wavy grooves. If 
this occurs it will be necessary to turn down the com- 
mutator and collector. 



Magnetic Cscillators 
No.2 






VolUgB 



M 



Interrupter 

Fig. 178. Diagram of Connections for Magnetic Oscillator. 



Figs. 170 and 171 show views of the Allis-Chalmers 
rotary converters, and the following is a brief descrip- 
tion of their construction: 

The field yoke, which is of the best quality of cast iron, 
is divided horizontally, and securely bolted together by 



ROTARY CONVERTERS 385 

fillister head screws, fitting into countersunk pockets cov- 
ered by the name plates. This gives the machine a grace- 
ful and pleasing appearance. 

The solid cast steel poles are securely bolted to the 
yoke, and can easily be removed without disturbing the 
armature. They have extended pole tips designed so as 
to distribute the magnetic flux over a large number of 
armature teeth, thereby reducing the iron loss and heat- 
ing of that part of the machine, and also improving the 
commutation. 

The armature core is built up of thin discs punched 
from very soft and specially prepared steel, of uniform 
quality, and high permeability. After punching, these 
discs are thoroughly japanned to prevent eddy currents. 
Special attention has been given to the proper ventilation 
of the armature ; ventilating ducts are placed at frequent 
intervals throughout the core and the spider, and the ven- 
tilating spacers forming the ducts are designed so as to 
force a circulation of air through the armature. This 
results in low temperature and enables the machine to 
stand heavy overloads without injury. The coils are held 
in place by hardwood wedges. 

The collector rings are made of copper and are liberally 
proportioned. They are designed so as to insure cool 
operation and are thoroughly insulated from each other 
and from the ground. 

The armature coils are form-wound, and are absolutely 
interchangeable. The conductors are carefully insulated 
with the best material obtainable and the insulation is 
integral with the coil, no additional protection being re- 
quired in the slots. All armature coils are subjected to 
a very severe puncture test in addition to the full load 
test, which is made when the machine is assembled. 



386 



ELEMENTARY ELECTRICITY 



The commutator segments are of pure hard-drawn 
copper and are carefully insulated with specially selected 
mica that wears evenly with the copper. The commuta- 
tor is composed of a sufficient number of segments to 




Fig. 179. Brush-Rigging and Brush-Holders on Alternating Current 
End of Allis-Chalmers 500 K. W. Rotary Converter. 



keep the voltage between adjacent bars within conserva- 
tive limits, and is designed to give a liberal contact sur- 
face for the brushes ; sparkless commutation and freedom 
from flashing-over are thus secured. 

The brush holders on the alternating current end (see 



ROTARY CONVERTERS 38? 

Fig. 179) are of the tangential type ; copper gauze brushes 
are used, and the design of the brush holder is such that 
the pressure is uniform over the whole contact surface. 
The holders are attached to a cast-iron frame which i° 
fitted to the bearing pedestal, on the alternating current 
end. The brush holders on the direct-current end are of 
the standard Bullock patented type; they permit of easy 
adjustment and removal of the carbon brushes and are 
mounted on heavy brass studs, supported by a cast-iron 
spider attached to the magnet frame of the machine. 
Provision is made for rotating the brush yoke so that 
the brushes can be easily adjusted to the position of best 
operation. 

The bearings are designed with particular attention to 
large radiating surfaces. They are of the ball and socket 
type, and are both self -oiling and self-aligning. 

Rotary converters are built either shunt or compound- 
wound, as desired. For railway systems in particular, 
where the car units are large in comparison with the 
capacity of the converter, compound winding is desirable, 
as otherwise the voltage will drop considerably whenever 
a car is started. The compounding should, as a rule, be 
such that with the prevailing conditions of the transmis- 
sion line and transformers, regarding drop, reactance and 
regulation, the delivered voltage of the converters will be 
nearly constant for different loads and this voltage should 
be chosen as high as the car equipments will permit. 
There is of course no advantage in having a lower volt- 
age at light loads. 

In order to prevent the brushes from wearing grooves 
in the commutator, a slight oscillatory motion is imparted 
to the shaft, thus greatly prolonging the life of the com- 
mutator and keeping its surface in good condition. This 



388 



ELEMENTARY ELECTRICI'fY 



motion, which in belted machines is automatically pro- 
duced by the belt, is imparted to the armature shaft by 
the Allis-Chalmers oscillator. 




Fig. ISO. 



Brush-Yoke and Brush-Holders Used on Direct Current End 
of 500 K. W, Allis-Chalmers Rotary Converters. 



Hunting. The "hunting" of rotary conYerters has al- 
ready been alluded to, but not enlarged upon. A rotary 
converter, and the generator supplying it with current 
may be likened in their speed relations, to an engine fly- 
wheel driving: a second flvwheel bv means of a flexible 



ROTARY CONVERTERS §89 

coupling in which the driving force is transmitted through 
spiral springs. In this analogous mechanism the rotation 
of the second flywheel will be steady only as long as the 
rotation of the engine flywheel is steady; fluctuations in 
the rotation of the engine flywheel, will produce exag- 
gerated fluctuations in the rotation of the driven wheel, 
due to the flexible nature of the coupling between the 
two wheels. The action of this mechanism may be ex- 
plained more in detail as follows : 

If the engine flywheel momentarily increases its speed, 
the second wheel will not maintain the same position 
relative to the engine flywheel as would be the case with 
two wheels rigidly connected together. The inertia of 
the driven wheel will cause it to lag behind the other in 
relative position, until the resulting tension in the springs 
is great enough to overcome the inertia, and momentarily 
increase its speed, bringing it back to its previous relative 
position. At the same time the tension in the springs 
reacts upon the engine flywheel so that the speed of this 
wheel is momentarily decreased if the engine will permit 
of a reduction in speed. If the inertia of the driven fly- 
wheel is large, the springs may take it beyond its initial 
position to a position relatively ahead of that of the engine 
flywheel, in which case the springs will be compressed, 
causing the second wheel to fall behind again, thus start- 
ing an oscillation in the rotation of this wheel. To set 
up this oscillation, three conditions are necessary: first,. 
a variation in the regularity of the rotation of the engine 
flywheel ; second, a flexible connection between the two 
wheels, and third, inertia in the second wheel. 

In the electrical machines the action is very similar. 
If there is any irregularity in the speed of the generator 
the inertia of the armature of the rotary converter pre- 



390 ELEMENTARY ELECTRICITY 

vents it from instantly following the generator, and the 
result is a slight difference in the relative positions of the 
armatures of the two machines, thus causing a change in 
the phase positions of the generator E. M. F., and the 
converter counter E. M. F. This change in phase posi- 
tions causes a difference in the instantaneous values of 
the two E. M. F.'s which in turn causes a so-called cor- 
rective current to flow between the two machines, and 
this current in its action is equivalent to the tension in 
the coupling springs in the mechanical arrangement of 
the two flywheels. This irregularity in the rotation of 
the rotary converter is called hunting and the means for 
preventing it are as follows : 

The converter is provided with heavy copper grids that 
surround each pole face and extend across it imbedded in 
one or more slots. The form of these grids is shown in 
the illustration. The function of the copper grids is to 
act as dampers preventing the relative position of the 
converter armature from being changed by the corrective 
currents more than the initial change in the generator. 
The action is essentially a damping action and is the same 
as that of the copper magnet damper used in galvano- 
meters and is analogous to the action of a dash-pot on 
an engine governor. If all the coupling springs in the 
flywheel arrangement already considered were provided 
with dash-pots the controlling action of the dash-pots 
would tend to prevent the ''hunting" of the driven fly- 
wheel. 

STARTING ROTARY CONVERTERS. 

There are three usual methods of starting rotary con- 
verters. 

(a) By a separate alternating current starting motor. 



ROTARY CONVERTERS 39l 

(b) By applying direct current to the commutator, 
the converter starting as a shunt motor. 

'(c) By applying alternating current directly to the 
collector rings, the converter starting as an induction 
motor. 

(a) Startiiig with separate alternating-current motor. 
An induction motor is pressed on an extension of the 
armature shaft and this motor is used for bringing the 
armature up to synchronous speed. In order that the 
motor may bring the converter up to synchronous speed 
the motor is designed with a smaller number of poles 
than the converter and the resistance of the secondary 
rings is made such that when running the converter at 
synchronous speed and with normal field excitation the 
motor slip will be of such a value that the running speed 
of the motor will be the synchronous speed of the con- 
verter. It is not essential that this adjustment of slip be 
made accurately. If the speed is too high the slip may be 
increased by lowering the voltage or by increasing the 
load on the motor. The latter may be accomplished by 
connecting a rheostat across either the direct-current or 
alternating-current terminals of the converter. 

With four-pole rotary converters it is not practicable 
to use a two-pole induction starting motor on account of 
the large slip that would be required. With four-pole 
converters, therefore, a single-phase series motor is used. 

(b) Starting from the direct current side. For this 
method of starting direct current is obtained either from 
the regular service bus bars or from a separate generator 
or circuit and the converter is started as a shunt motor. 
If the converter is compound wound the series field coils 
must be short-circuited or the starting circuit must be 
connected inside of the series coils in order that changes 



392 ELEMENTAkY ELECTRICITY 

in armature current will not exaggerate the changes in 
speed by changing the field strength. 

The operation of starting is similar to that of any shunt 
fcnotor. The converter is started with strong field and 
resistance in the armature circuit provided by a starting 
rheostat. The converter is brought into synchronism 
With the alternating current circuit by adjusting the speed 
by the field rheostat, and when synchronism is reached the 
converter is parallel with the alternating current circuit 
by throwing in the a. c. switches in the high-tension or 
low-tension side of the transformers as the case may be. 

(c) Starting from the alternating current side. In 
this method the alternating current side of the converter 
is connected to the alternating current circuit, the direct 
current switches being open. Under these conditions the 
rotary converter starts as an induction motor, the arma- 
ture acting as the primary and the field poles as the sec- 
ondary. The field winding plays no part in starting, as 
it is open circuited in several places, by means of a break- 
up switch usually located on the side of the frame of the 
machine. It is necessary to open up the field winding into 
several sections in order to limit to a safe value the volt- 
age induced in it by the rotating magnetic field set urj by 
the polyphase current in the armature winding. This 
method is objectionable, and not to be pursued when it 
can be avoided. 



SYNCHRONIZING ROTARY CONVERTERS. 

When a rotary converter is started from the direct 
current side or by a separate starting motor, it must be 
in synchronism with the alternating current bus bars be- 
fore it can be connected to them. Two machines, a gen- 



ROTARY CONVERTERS 



393 



erator and rotary converter, for example, are said to 
be in synchronism when their frequencies are the same 
and when their phase is the same. The two machines will 
have the same frequency when the numbers of alterna- 
tions, or reversals of their E. M. F.'s in a given time are 
equal. This condition will be fulfilled when the product 
of the number of poles by the revolutions per minute for 
each machine is the same. The two machines will have 
the same phase when the positions of the armatures with 
respect to the field poles are the same, i. e., when similar 
armature coils are opposite positive field poles at the same 
instants. In addition to being in synchronism with the 




Fig. 181. Lamp Method of Synchronizing. 



generator to which it is to be connected, the rotary con- 
verter must have approximately the same voltage as the 
generator as measured by a voltmeter. 

The operation of synchronizing a rotary converter with 
a generator consists in bringing the rotary converter up 
to approximately synchronous speed and to the same volt- 
age as the generator, and then when the inevitable fluc- 
tuation in the speed of the two machines brings them in 
phase, in connecting them together. When the two ma- 
chines have the same frequency, the same phase, and the 



394 ELEMENTARY ELECTRICITY 

same voltage, there will be no unbalanced E. M. F. and 
consequently no rush of current when the machines are 
connected together. 

In synchronizing two machines it is evidently necessary 
to have some means for determining when the frequency 
and phase position of the incoming machine are right. 
The principle of the most common method of doing this 
is illustrated in Fig. 181. 

A and B represent two single-phase machines or a 
single phase each of two machines, the leads of which are 
connected to the switch C. through two series of incan- 
descent lamps, D and E. It is evident that as the relative 
positions of the phases of the electromotive forces change 
from that of exact coincidence to that of exact opposition 
the flow of current through the lamps varies from a 
minimum to a maximum. If the electromotive forces of 
the machines are in phase and of the same value, the cur- 
rent through the lamps will be zero. But as the difference 
in phase increases, the lamps will light up and increase 
in brilliancy until the maximum is reached, when the 
phases are in exact opposition. From this position .they 
will decrease in brilliancy until dark indicating that the 
machines are in phase again. The rate of pulsation of 
the lamps depend on the relative frequency of the ma- 
chines to be synchronized. If this rate is reduced to about 
one pulsation in ten seconds, ample time is allowed for 
closing the circuit. 

The ideal synchronizer should perform three distinct 
functions : 

(i) It should indicate whether the incoming con- 
verter is running too slow or too fast. 

(2) It should indicate the amount by which an incom- 
ing converter is too fast or too slow. 



ROTARY CONVERTERS 395 

(3) It should indicate the exact time of synchronism 
or coincidence in phase between the bus bars, and the 
incoming converter. 



THE WESTINGHOUSE SYNCHROSCOPE. 

This instrument is said to perform all of the above 
functions with perfection. The phase angle between the 
converter and generator is always equal to the angle 
between the pointer and its vertical position marked on 
the dial of the instrument. If the frequency of the incom- 
ing machine is higher than that of the line, this angle 
will vary, causing the pointer to rotate in one direction. 
If the incoming machine is lower in frequency, the 
pointer will rotate in the opposite direction. Fast and 
slow on the dial mean that the frequency of the rotary 
is higher or lower than the frequency of the line. When 
the frequency of the converter equals that of the line the 
pointer stops at some position on the scale, and when 
the converter is in phase with the line, the pointer coin- 
cides with the dummy pointer at the top of the scale. The 
main switch may then be closed. Two shunt transform- 
ers are necessary to connect the instrument to the bus 
bars, and one additional shunt transformer to connect it to 
each machine to be synchronized. When installed, the 
same shunt transformer may be employed in connecting 
other instruments to the bus bars and machines. 



THE AUTOMATIC SYNCHRONIZER. 

The uncertainty in synchronizing which arises from the 
hand-throwing of switches is done away with by the 
automatic synchronizer. 



396 ELEMENTARY ELECTRICITY 

The instrument consists essentially of two solenoids, 
the upper ends of whose movable cores are flexibly con- 
nected to either end of a cross-beam pivoted at its center 
as an ordinary walking-beam. These solenoids are so 
connected that the one receives a maximum current at 




Fig. 182. View of Automatic Synchronizer, Case On. 

the instant of synchronism and the other receives a min- 
imum current at the same instant. To accomplish this, 
the right-hand solenoid is connected in the same manner 
as a synchronizing lamp is connected to synchronize light, 
and the left-hand solenoid is connected like a lamp to 
synchronize dark. 

Attached to the shaft of the cross-arm is a small con- 
tact finger or clip. This contact device is for closing a 
circuit through the relay switch which closes the circuit 
through the closing coil of an electrically operated switch 



ROTARY CONVERTERS 



397 



at the proper moment of synchronism. The current for 
actuating the switch is taken from a source independent 
of the generators, such as the exciter. To the. cross-beam 
is also attached one element of a dash-pot. The other 
element is connected through a system of levers to a disc 
of insulating material mounted on a short shaft in line 







Fig. 183. View of Automatic Synchronizer, Case Off. 



with the pivot of the cross-beam. A small metal seg- 
ment mounted on the disc is a little longer than the gap 
between the movable clip and a stationary clip, when the 
clips are at their minimum distance apart. Mechanical 
adjustment is made such that this minimum distance point 
is reached coincident with the point of synchronism. Ref- 
erence to Fig. 183 will show how this dash-pot action on 
the disc prevents the movable clip from making contact 



398 ELEMENTARY ELECTRICITY 

with the stationary clip when the rocking motion of the 
crossbeam is too rapid. Before the incoming machine 
has approached synchronism both of the solenoids are 
acted upon equally by currents from the synchronizing 
transformers and the cross-beam will assume a position 
midway between its two extreme positions. As the point 
of synchronism nears, the beam will begin to oscillate, 
following in its movements the variations in the currents. 
In one solenoid the current is a maximum while in the 
other it is a minimum, and vice versa. As soon as the 
oscillation becomes slow enough, the dash-pot is pulled 
out to its maximum length with the forward movement 
of the beam, and the contact piece on the insulating disc 
remains in the proper position to make the circuit be- 
tween the moving and the stationary clips. 

If the voltage of the incoming machine differs consid- 
erably from that of the bus bars to which it is to be con- 
nected, the device will not close the contact since the 
effect of the excessive voltage on the left-hand solenoid 
is to hold that end of the beam too low at the moment 
of synchronism. It is thus seen that the incoming ma- 
chine will not be thrown in unless the voltages are ap- 
proximately equal, the machine is in phase with the line, 
and the frequency is right. 

A controller and relay switch are interposed in the cir- 
cuit with the synchronizer and the electrically operated 
switch. By means of the controller switch the main or 
electrically operated switch may be tripped, but cannot 
be closed, as the closing coil is normally out of circuit, 
until the synchronizer is in the position assumed at the 
synchronous operation of the generators, or rotary con- 
verters, that is, when the solenoid of the relay is ener- 
gized, and the contacts closed, 



ROTARY CONVERTERS 399 

This closes the relay switch circuit, and completes the 
path of the current through the controller and electrically 
operated switches. The relay switch is provided with 
carbon break and relieves the contacts of the synchroni- 
zer from excessive currents. 

For rotary converter work a simple form of electrically 
operated switch has been devised for use with the auto- 
matic synchronizing system, combining the functions of 
a switch and an automatic circuit-breaker, thus omitting 
the knife switches and fuses. As automatic switches 
require a certain period of time to close, it is necessary 
that the closing action begins in advance of the point of 
synchronism, in order that the switch will close at the 
correct time. This adjustment is easily made to suit anv 
particular form of electrically, or pneumatically operated 
switch. The wiring of the various devices forms a com- 
plete interlocking system, preventing the coupling of ma- 
chines under unfavorable circumstances, and avoiding the 
possibility of any resulting damage. 



SYNCHRONIZING POLYPH'ASE CONVERTERS. 

What has been said regarding the synchronizing of two 
single-phase circuits applies equally well to the synchron- 
izing of the individual, and similar circuits of polyphase 
rotary converters. 



PARALLEL OPERATION OF ROTARY CONVERTERS. 

Direct current side. If several rotary converters are 
to supply current to the same direct current system they 
may be connected in parallel just like shunt or compound 
wound direct current generators. With compound wound 



400 ELEMENTARY ELECTRICITY 

converters an equalizing connection must be' made be- 
tween the different machines. 

The load will be divided among the different converters 
according to their direct current voltages, as in direct 
current practice. The voltage may be regulated by chang- 
ing the alternating current voltage as will be hereinafter 
described ; or by regulating the field current, which 
changes the direct current voltage by increasing the drop 
in voltage in the armature; or by shifting the direct cur- 
rent brushes which changes the direct current voltage by 
collecting less than its maximum. 

When one of two machines having strong series fields 
is carrying load, it may be found necessary in paralleling, 
to short circuit the series coils while throwing the ma- 
chines together. The machine thrown in has its field 
strengthened by dividing up the load, and the other ma- 
chine is weakened, thereby causing the flow of large cur- 
rents between the machines. This extra current is liable 
to cause a disturbance which may reverse the polarity of 
the machine carrying the load as well as to cause a heavy 
rush of current in the alternating current feeder circuits. 

Alternating current side. It is not advisable to have 
different rotary converters, supplying current to the same 
direct current system, connected together on the alter- 
nating current side, i. e., supplied from the same trans- 
former secondaries. When this is done, a complete local 
circuit is formed by two converters and the alternating 
current, and direct bus bars, and any difference in the con- 
ditions of operation in the two machines — a slight differ- 
ence in the relative positions of the direct current brushes 
of the two machines, for example — will cause large cross 
currents in this local circuit. The converters should be 
supplied by separate transformers or by separate sec- 
ondaries when a single set of transformers is used. 



ROTARY CONVERTERS 40t 



GENERAL INSTRUCTIONS. 

(i) When the converters flash over or the breakers 
come out due to excessive current, it is always wise to 
note the d. c. voltmeter before throwing in on the line 
again, as these troubles very frequently cause a reversal 
of polarity in the fields, making them build up in the op- 
posite direction. If this should be the case, it will be 
necessary to reflash same as instructed elsewhere. 

(2) When the a. c. power goes off for any reason^ 
shut down the converter at once, opening all switches. 
By closing the synchronizing plugs the lamps will indi- 
cate when the power comes on again. 

(3) When the a. c. breakers come out, open the d. c. 
breaker and switches, and then proceed to synchronize as 
in first starting. 

(4) When a converter flashes over and is thrown out 
of circuit, it is best if possible to shut down for a moment 
and examine the commutator to clean up any burs which 
may have been caused on same. If this is not possible the 
commutator can be cleaned after the converter has been 
put in service by exercising the proper amount of care. 

(5) Cleanliness is an important factor in the proper 
operation of all electric machinery, and proper care should 
be exercised on this point. 

INSTRUCTIONS FOR OPERATION. 

To Start a Single Converter with a Starting Motor. 

(1) See that both the a. c. and the d. c. brushes are 
properly adjusted, and that everything is clear about the 
converter. 



4.2 ELEMENTARY ELECTRICITY 

(2) See that all switches on the board are open oh 
both the a. c and d. c sides, and that the resistance of 
the rheostat is all cut in the field circuit. 

(3) Close the circuit breakers on the a. c panel. 

(4) See that the plugs to the synchronizing lamps, 
or synchroscope are in. and that the lamps burn dimly; 
also that the plug for the d. a voltmeter is in. 

(5) Start the rotary by closing the starting motor 
switch. 

(6) Examine all oil rings on the converter t: 
that they are turning, and carrying oil. 

(7) Build up the <L c. voltage to approximately the 
line voltage by cutting out resistance in the rheostat. 

(8) Close the main a. c switches to within an inch 
or so of the jaws, so that they may be thrown in quickly 
at the proper time. 

(9) Note the lamps, and adjust the speed by means 
of the rheostat until the lamps dim slowly and regularly. 
When the lamps indicate the proper phase relation, (pre- 
ferably when they are dark) close the a. a switch. It is 
better to time your decision so that the switch will be 
closing as the lamps become dark, rather than when the 
converter is receding from synchronism. 

It is sometimes necessary to shift the d. c. brushes back 
from the neutral point in order to get the proper speed. 

( 10) Open the starting motor switch. 

(11) After the converter is synchronized adjust the 
field rheostat so that you get minimum current in the 
a. c ammeters. 

( 12) Close the d. c breaker and switches, and adjust 
the brushes to a non-sparking position. 



Rotary converters 403 

To Start a Single Converter from the Direct Current 

Side. 

Assuming the circuit breaker on the positive side, and 
the starting resistance across the negative switch. 

(i) See that all switches are open and that the d. c. 
brushes are properly set, either on, or a little back from 
the neutral point. It is better to start with the series coil 
reversed, or at least cut out altogether. 

(2) Close the a. c. breakers and synchronizing plugs. 

(3) Close the d. c. breaker and the positive switch. 

(4) Close the field switch having the resistance all cut 
out, and try the field poles to see that they are magnet- 
ized. 

(5) Start the converter with the starting resistance, 
by cutting out same slowly. Close the negative switch. 
There should be in addition to the starting resistance a 
permanent resistance in the starting circuit. 

(6) Examine oil rings to see that the same are turn- 
ing, and carrying oil. 

(7) Regulate the speed to the synchronous speed of 
the generator by means of the rheostat and synchronizing 
lamps. It is desirable though not necessary to have the 
a. c. voltage of the converter the same as that of the line. 
You may be aided to a slight extent in this adjustment 
by moving the brushes. 

(8) When the voltage is correct and the lamps indi- 
cate the proper phase relation, close the a. c. switches. 

(9) Adjust the rheostat for the minimum a. c. current 
and set the d. c. brushes at a non-sparking position. 



404 ELEMENTARY ELECTRICITY 

To Start a Second Converter, to be Run in Parallel with 
Another. 

( 1 ) In starting a second converter to be run in paral- 
lel with another, follow the same procedure as in starting 
a single converter, using the synchronizing and meter 
plugs on the panel of the converter to be started. 

(2) In building up the d. c. voltage make sure that 
it builds up in the same direction as that of the other 
rotaries, and the brush holders are in a similar position. 

(3) Synchronize as usual by means of the lamps, and 
after having thrown the machine in, open the starting 
motor switch. 

(4) In paralleling the d. c. side it is best to close the 
switches when the load on the first converter is a mini- 
mum. If the load on the converter having a strong series 
field is constant and heavy, it is better to short-circuit the 
series field on both converters, while throwing them to- 
gether, otherwise the sudden dividing of the load may 
cause a rush of current between the machines and cause 
a reversal of polarity in the first converter. 

(5) Regulate the a. c. by means of both rheostats to 
obtain the minimum. 

(6) The proportions of the load may be regulated to 
a slight extent by shifting the d. c. brushes on the con- 
verters. 

What to do When the Circuit Breaker Comes Out. 

( 1 ) If while operating, the d. c. circuit breaker comes 
out, first note the breakers and meters on the a. c. side, 
and see if everything is all right there. If so, open the 
switch on the same circuit with the d. c. breaker, or if the 
switches are connected, open all, then close the breaker 



ROTARY CONVERTERS 40o 

and afterward the switches. If the breaker continues to 
come out, have the trouble located and remedied. 

(2) If only one a. c. breaker comes out and the 
machine stays in step it may be possible to open the 
switch under the same, and throw the circuit breaker in 
again quickly if everything else is right. It is advisable 
to note the ammeter on this circuit after closing same, to 
see if the line is open. 

(3) If the a. c. breakers come out and the machine 
falls out of step open at once the d. c. breaker and all 
switches. It will then be necessary to synchronize the 
converter in the same manner as when starting. 

(4) If two or more converters are running in paral- 
lel, and one falls out of step, first open the d. c. breaker 
on this converter to prevent the direct current from the 
other from feeding back. Then open all switches and 
synchronize as usual. 



INVERTED ROTARY CONVERTERS. 

A rotary converter running from direct current to al- 
ternating current has no longer a constant speed. When 
a rotary is driven by direct current, its speed follows the 
law of a direct-current motor and is governed by the 
strength of the field and therefore by the reaction of the 
armature current upon the field. When the alternating 
current load is inductive, changes of load may result in 
considerable variation in speed and, the converter is liable 
to reach an excessive speed if there is an application of 
a heavy inductive load. This may, however, be overcome 
■by exciting the converter from a generator, the speed of 
which varies with that of the converter, particularly if the 
field of this generator is unsaturated at normal voltage. 



406 



ELEMENTARY ELECTRICITY 




ROTARY CONVERTERS 407 

Then any change in the speed of the rotary converter will 
cause a much greater change in the exciting voltage, and 
the field of the rotary converter will be changed. Thus 
the speed of the rotary will be independent of the amount 
or nature of the load. This is the method adopted by 
the Westinghouse Company. 

When this method of exciting an inverted rotary con- 
verter is used, the converter is of the same size and pro- 
portion as a standard rotary converter of the same capac- 
ity. When self-excitation is used the inverted converter 
must be of larger size than a corresponding standard 
converter, so that the armature reaction may be made very 
small. There must also be provided an automatic Fpeed- 
limiting device that will interrupt the power circuit in case 
the converter speed reaches a dangerous value. 

With the inverted rotary converter one rheostat is con- 
nected in the converter field, and another in the exciter 
field. The rheostat in the converter field is used only in 
starting, and should be so connected in the circuit that 
when the converter is excited from the separate exciter, 
this rheostat is entirely out of circuit. The converter 
field should be connected to the exciter terminals with no 
resistance between them. 



TRANSFORMERS. 

In Fig. 185 is shown the principle upon which the 
transformer used in alternating current work is operated. 
Two separate coils of wire are wound on a ring of lami- 
nated iron. One of the coils contains a number of turns 
of fine wire, while the other contains only a few turns 
of large wire. When an alternating current is sent around 
the coils of fine wire, generally called the primary, a cur- 
rent will be induced in the coil of heavy wire, or second- 
ary. The amount of current induced in the larger wire 
will be relatively greater in amperes and less in potential 
than that of the fine wire circuit. This ratio is almost 
entirely dependent upon the relative number of turns 
existing between the large and small wires. 





H 

Fig. 185. Diagram of Alternator, Line, Transformer, and Secondary 

Circuit. 



Fig. 185 A represents the alternator, B its brushes and 
D and E the mains to the transformer H. This trans- 
former consists of a core of iron C on which are two wind- 
ings. The coil P is called the primary, and is connected 
to the main from alternator. The other coil S is called 
the secondary, and to it the load is connected. 

Whatever the voltage of alternator A, that of the sec- 

408 



TRANSFORMERS 



409 





Fig. 186. Transformer Coils in Wound, Bound and Taped Stages o{ 
CompletioD. 



410 



ELEMENTARY ELECTRICITY 



ondary circuit F. L. G. will be three-eighths of it because 
there are 8 turns on primary and 3 turns on secondary. 
The power in the secondary circuit is practically the same 
(minus the losses) as is given out by the alternator, hence 
the primary current is low and wire is small. The sec- 
ondary current is large so wire is large. 

Since one kilowatt can be a combination of a large 
current and small pressure, or small current and large 
pressure, it is evident that the transformer simply trans- 
fers the power, and transforms the voltage and indirectly 
the current. 




Fig. 187. Coils, Air Ducts and Separators for Transformer. 



This transformer (Fig. 185) lowers the voltage and is 
called a step down transformer. 

When the secondary is connected to the alternator, the 
transformer raises the voltage and is called a step up 
transformer. 

The coils of a transformer must be very well insulated. 
After winding they are bound to keep them in shape, and 



TRANSFORMERS 



411 




jF*- .. 




Fig. 188. Interior Construction of an Air Blast Transformer. 




Fig. l§9r Set pf Coils Made Up Rea<ty to Be Placed is IrassformeFt 



412 ELEMENTARY ELECTRICITY 

then wound with linen tape, or varnished cambric cloth. 
Fig. 1 86 shows a coil in the three stages of completion. 

In Fig. 187 is shown a set of completed coils, together 
with the ventilating ducts and mica barriers sufficient for 
one leg of a transformer. 

Fig. 188 shows the two legs of a transformer, which 
form its iron core, each over half-filled with coils. The 
coil is made of sheets of soft iron. 

Fig. 189 shows the manner in which the coils are 
sometimes bound up to be placed in transformer as one 
coil. 

EXCITING CURRENT. 

The Exciting Current, being also called by various 
other names, such as leakage current, open circuit cur- 
rent, and magnetizing current, is a very important factor. 

In order that a transformer may be ready to do its 
work it is always connected to the line. This means that 
the primary coil is always magnetizing the core if no 
current is drawn from the secondary. 

This steady flow of current to excite the primary is 
the price we have to pay for having the transformer 
continually ready, for service. 

A transformer should therefore never be left on a line 
unless it is needed. 



EFFICIENCY OF TRANSFORMERS. 

v 

1 

The losses in transformers are less than any other 
piece of electrical machinery or apparatus ; 98% of the 
intake being delivered in the larger sizes as used in rail- 
road sub-stations or power houses, when fully loaded. 



TRANSFORMERS 413 

Unfortunately they lose about the same amount of power 
at all loads. 

A. ioo K. W. transformer loses 2 K. W. at full load, 
its efficiency is then 98-^100=0.98. At half load it loses 
2 K. W. but is only carrying 50 K. W. so (its losses are 
now equivalent to 4 K. W. on a 100 K. W.) its efficiency 
is 48-^50=0.96. 

At quarter load it takes in 25 K. W., loses 2 K. W., 
so its efficiency is 23^-25=0.92. 

By clever designing transformers are built to be most 
efficient at three-quarters load. They are a little less 
efficient at half and full loads, and still less at quarter 
load and quarter overload, but never fall below 95%. 



COOLING TRANSFORMERS. 

Small transformers hung up on poles are cooled by 
surface radiation only. 

Medium, sized ones are filled with oil. This conducts 
the heat to the iron case, and also acts as an insulator. 

The oil will also flow in and fill a break in the cloth or 
mica after a puncture. 

Air blast avoids the danger of oil in case of fire or 
flame due to short circuits. They are cheap as a trans- 
former may be much more heavily loaded when cooled 
by the air blast, and the blower only consumes 1/10 of 
1% of the full load output of transformer. 

Fig. 190 shows the interior construction of an air blast 
transformer and Fig. 191 shows how they are installed. 

Water cooled. These are the smallest and cheapest 
transformers to build but not so cheap to run as the air 
blast. 



414 



ELEMENTARY ELECTRICITY 



The cases are rilled with oil which absorbs heat from 
coils. Pipes are run through the oil, in which cold water 
is circulated. 




Fig. 190. Air Blast Transformer. 



In a water power plant where the head of water would 
render pumps unnecessary the water cooled type would 
certainly be the best, 



TRANSFORMERS 



415 



□ 



l! 



CO 

co 
ca 

CO 
CO 



/% 



< o 

g o 

3 3 



a h 

a ^ 

CO 



416 ELEMENTARY ELECTRICITY 



AUTO-TRANSFORMERS. 

These are only applicable to certain cases. 

The idea is shown in Fig. 192. The same coil of wire 
A to B is used as primary and secondary, the whole be- 
ing the primary, and portions as C to D, D to E, or C to E 
being used as secondary. 

They are only used where the primary voltage is fairly 
low and the secondary voltage is not less than one-fifth 
of the primary voltage. 

They are used instead of resistances to start A. C. 
motors. 




B E 

Fig. 192. Diagram of Auto-Transformer or Compensator. 
ALLIS-CHALMERS POWER TRANSFORMERS. 

Transformers for use on power transmission lines are 
made by Allis-Chalmers Company in three different types, 
depending on the method used for cooling. These types 
are as follows: oil-filled self-cooled (O. F. S. C.) ; oil- 
filled water-cooled (O. F. W. C.) and air-blast. In the. 
first the heat is carried off by radiation and conduction 
from the case, in the second by the circulation of water 
through coiled pipes immersed in the oil, and in the third 
by currents of air forced through the transformer. Oil- 



TRANSFORMERS 417 

Insulated transformers are used in the great majority of 
cases, and the question as to whether they shall be self- 
cooled or water-cooled is determined largely by the size of 
the units and the available supply of cooling water. 




Fig. 193. Allis-Chalmers Oil-Filled Self-Cooled Transformer, 60 Cycle, 
170 K. W., 20,000 to 2,300 Volts. 

Self-cooled transformers are built in sizes up to 250 K. 
V. A. (kilovolt-amperes). Above this size the external 
surface of the case is not sufficient for the effective radia- 



41S 



ELEMENTARY ELECTRICITY 




Fig. 194. 



Allis-Chalmers Oil-Filled Water-Cooled Transformer, 
Cycle, 500 K. W., 20,000 to 375 Volts. 



25 



TRANSFORMERS 419 

tion of the heat unless the case is made abnormally large. 
Fig. 193 shows a standard transformer of this type. 

Water-cooled transformers, Fig. 194, are made in sizes 
from 100 K. V. A. up. Water is circulated through a 
coil of seamless copper tubing immersed in the oil in the 
upper part of the tank, and the heat is effectively carried 
off. Wherever water is available and not expensive this 
method is preferable to air cooling, even for compara- 
tively small transformers, as it permits operation at lower 
temperatures, and allows more margin for overloads. 

Air-blast transformers are made in sizes from 75 K. V. 
A. up. Cooling is effected by placing the transformer 
over an air chamber in which a pressure of air is main- 
tained by motor driven fans; currents of air passing up 
around the transformer carry off the heat. In this type 
oil cannot be used for insulating purposes, and 25,000 
volts is the highest pressure for which it is advisable to 
build them. 

Much has been written about the relative fire risks of 
air-blast and oil-filled transformers, but this is a matter 
that depends as much on surrounding conditions, and the 
location of the transformers as upon the construction. 
The air-blast transformer contains a small quantity of 
inflammable matter as compared with the oil-filled trans- 
former, but this material is much more easily ignited. A 
breakdown in an air-blast transformer is usually followed 
by an electric arc that sets fire to the insulating materials, 
and the flame soon spreads under the action of the forced 
circulation of air. Although the fire is of comparatively 
short duration, it is quite capable of igniting the building 
unless everything near the transformers is of fire-proof 
construction. 

The chance of an oil-filled transformer catching fire 



420 



ELEMENTARY ELECTRICITY 



on account of any short circuit in the windings is ex- 
tremely small, because oil will burn only in the presence 
of oxygen, and, since the transformer is completely sub- 
merged in oil, no air can get at it. Moreover, the oil 




Fig. 195. High Voltage Coils. 

used in transformers is not easily ignited ; it will not burn 
in open air unless its temperature is first raised to about 
400 F. In fact, the chief danger of fire is not that the 
oil may be ignited by any defect or arc within the trans- 
former, but that a fire in the building in which the trans- 
formers are installed may so heat the oil as to cause it to 
take fire. 



Transformers 



421 



General construction. The general construction of the 
6oils and core is much the same in all three types, regard- 
less of the method of cooling. All transformer coils are 
Wound With double cotton covered strip copper, one turn 




Fig. 196. Low Voltage Coils. 



per layer, with fullerboard insulation, in addition to the 
cotton covering, between turns. Exceptions to this con- 
struction are made only when the size of the conductor 
is such as to render the use of copper strip impracticable, 



ELEMENTARY ELECTRICITY 




Fig. 197. Method of Spacing High Voltage and Low Voltage Coils, 



TRANSFORMERS 



423 




Fig. 198. Partly Assembled 25 Cycle, 500 K. W., 20,000 to 375 Volt 
Transformer. 



424 



ELEMENTARY ELECTRICITY 




Fig. 199. Core and Coils, 500 K. W., 20,000 to 375 Volt Transformers. 



TRANSFORMERS 



425 





426 ELEMENTARY ELECTRICITY 

and in such cases the coils are wound with round double- 
cotton covered wire with few turns per layer, so that the 
voltage between the layers is kept within safe limits. The 
core is built up of steel sheets 0.0014 inch thick. In the 
larger sizes, space blocks are placed every few inches in 
the core, thus providing ducts through which oil can 
circulate and carry off the heat. Also, in assembling the 
coils, spaces are formed between the coil sections, and 
between the coils and core, so that all parts are in contact 
with a free circulation of oil. Fig. 198 shows a core 
partly built up and Fig. 199 core and coils completely 
assembled. Fig. 200 shows the windings and core for a 
transformer of smaller output. 



QUESTIONS. 

1. How is direct current transformed from one volt- 
age to another ? 

Ans. By means of a machine called a motor-generator. 

2. Describe in brief a motor-generator. 

Ans. It consists usually of a d. c. motor driven by 
current at the voltage of the incoming line. This motor 
in turn drives a d. c. generator that furnishes currrent 
at the desired voltage. 

3. How is the outgoing voltage regulated? 

Ans. By altering the field strength of the generator. 

4. In case the incoming and outgoing current can bear 
the same ratio to each other constantly, what kind of an 
apparatus is used? 

Ans. A machine called a dynamotor. 

5. Describe the operation of a dynamotor? 

Ans. It is a d. c. motor running on the incoming volt- 
ages. On the same armature core is a separate winding 
connected to its own commutator at the other end of the 
armature. One set of field magnets serves for the motor 
winding and the generator or dynamo winding. 

6. Describe in general terms a rotary converter? 
Ans. It combines in a single machine the functions 

of a motor-generator and a dynamotor. 

7. Why are rotary converters and transformers 
necessary? 

Ans. Because it is more economical to transmit alter- 
nating current at high voltages and transform, or convert 
it to the lower voltage at which it is used. 

427 



428 ELEMENTARY ELECTRICITY 

8. Give another reason for using rotary converters? 
Ans. For the purpose of transforming alternating 

current into direct current when direct current is used. 

9. What is the chief point of difference between a 
rotary converter and a direct current generator? 

Ans. The rotary has collector rings connected to cer- 
tain points of the armature winding. 

10. What governs the number of such connections? 
Ans. The number of poles and phases. 

11. Describe the different types of rotaries? 

Ans. They are built for single-phase, two-phase, 
three-phase or six-phase. 

12. How many collecting rings has a two-phase con- 
verter? 

Ans. Four collecting rings. 

13. How many collecting rings has a three-phase 
converter ? 

Ans. Three collecting rings. 

14. When alternating current is transmitted at high 
pressure, what means are employed for lowering the po- 
tential ? 

Ans. Transformers. 

15. When the incoming current is direct and the out- 
going current alternating, how is the voltage raised? 

Ans. By step up transformers. 

16. Describe the winding of a rotary converter? 
Ans. It is usually shunt wound, or compound wound, 

although sometimes separately excited. 

17. How are rotaries in railway service usually 
wound ? 

Ans. Compound, owing to variations in the load. 

18. W'hat advantage is gained by this method of 
winding? 



QUESTIONS AND ANSWERS 429 

Ans. It tends to maintain the d. c. voltage constant. 

19. Upon what does the ratio between the a. c. and 
d. c. voltages of a rotary depend? 

Ans. Upon the number of phases, the lead given the 
d. c. brushes, the wave form of its alternating current, 
and upon the field excitation. 

20. Does the armature drop affect this ratio to any 
extent ? 

Ans. It does by decreasing it slightly when running 
a. c. to d. c. and increasing it when running d. c. to a. c. 

21. What are the ratios of conversion approximately? 

Ans. Single-phase 71 

Two-phase 71 

Three-phase 61 

Six-phase 71 or .61 

22. Give an example illustrating above. 

Ans. If d. c. voltage is 550 volts, the a. c, if two- 
phase will be 500 X. 7 1 =390 volts, or if three-phase it will 
be 55<>X.6i=335 volts. 

23. What precautions should be observed in the erec- 
tion of a rotary converter? 

Ans. First — It should be protected from moisture. 
Second — It should be protected from dust or dirt. Third 
— It should be in a well ventilated room and kept as cool 
as possible. 

24. Should the frame of the machine be insulated ? 
Ans. Generally speaking the strain on the winding 

insulation will be decreased, and danger to attendant in^ 
creased by insulating the frame. 

25. If a rotary has been exposed to dampness how 
may it be dried out? 

Ans. By running it with about 10 per cent of the 



430 ELEMENTARY ELECTRICITY 

normal a. c. voltage, while at same time observing cer- 
tain precautions noted in the text of this boolc under 
head of rotary converters. 

26. What method should be pursued in caring for the 
commutator 

Ans. Wipe it off with a piece of canvas — never use 
waste. Lubricate it with a very small quantity of vase- 
line, or oil applied with a piece of cloth. See that none 
of the segments is at all loose. 

If it gets out of true turn it down. 

2J. If a commutator gets hot while carrying only a 
normal load what should be done? 

Ans. Heating under such conditions is an indication 
that the commutator is worn out, and should be replaced 
by a new one. 

28. Give some of the causes of sparking at the 
brushes. 

Ans. Brushes may not have proper lead. 
Brushes may not fit commutator. 
Brushes may be burned on end. 
Commutator surface may be rough. 

29. What is meant by a rotary bucking? 

Ans. When arcing occurs between two adjacent brush 
holder arms, thus short circuiting the machine. 

30. Name a few of the principal causes of bucking. 
Ans. Rough or dirty commutator. 

Excessive voltage. 
Fluctuations in the voltage. 

31. What is an oscillator, and what is its function? 

Ans. An oscillator is a device operated either mag- 
netically, or by mechanical means, and its function is to 
produce a slight, periodic movement of the armature 
shaft endwise. 



QUESTIONS AND ANSWERS 431 

32. Why is this endwise movement of the shaft nec- 
essary? 

Ans. In order to prevent the wearing of grooves in 
the commutator. 

33. What is meant by the hunting of a rotary con- 
verter? 

Ans. It is a slight change of the speed of the arma- 
ture. 

34. What is the cause of hunting? 

Aiis. Irregularities in the speed of the generator de- 
livering current to the rotary, thus causing a slight dif- 
ference in the relative positions of the armature of the 
two machines, resulting in a change in the phase posi- 
tions of the generator E. M. F. and the counter E. M. F. 
of the converter. 

35. What are the usual methods of starting rotary 
converters ? 

Ans. First — By a separate a. c. starting motor. 

Second — By applying direct current to the commuta- 
tor. This starts the converter as a shunt motor. 

Third — By applying alternating current directly to the 
collector rings. This starts the converter as an induc- 
tion motor. 

36. What is meant by synchronizing a rotary conver- 
ter? 

Ans. Bringing it to the same frequency, the same 
phase, and the same voltage as the generator from which 
it is receiving current. 

37. What method is employed to determine when the 
machines are in synchronism? 

Ans. There are several methods, the most common 
one being by means of incandescent lamps connected in 
series with the two machines. 



432 ELEMENTARY ELECTRICITY 

38. What is a synchroscope? 

Ans. It is an instrument for determining v/hen elec- 
trical machines are in synchronism. 

39. What is an automatic synchronizer? 

Ans. It is a device that will automatically synchro- 
nize two electrical machines; also connect synchronized 
machine with the main by means of an electrically op- 
crated switch. 

40. Name two important points to be looked after be- 
fore starting a rotary converter. 

Ans. First — See that both the a. c. and d. c. brushes 
are properly adjusted and that every thing is clear about 
the converter. Second — See that the switches on board 
are open on both the a. c. and d. c. sides, and that the 
resistance of the rheostat is all cut in the field circuit. 

41. What is the function of a transformer? 

Ans. To transform the current from a higher, to a 
lower voltage, or vice versa. 

42. What principles govern the action of a trans- 
former ? 

Ans. The principles of electro-magnetic induction. 

43. What is a step up transformer? 

Ans. A transformer that raises the voltage. 

44. What is a step down transformer? 
Ans. One that lowers the voltage. 

45. How are transformers cooled? 

Ans. Small sizes by surface radiation. Larger sizes 
by oil ; also by air blast. Some of the smaller sizes are 
cooled by water circulating through surrounding coils. 



INDEX. 

A 

A-C current 282-356-358 

Action of electric condensers . . ; 48 

Amalgam for storage battery ...;..:..; 248 

Ampere 1 . . 1 . * . . » * . 1 . < . « 300 

hour ;....,.<.. 300 

Anode " 276, 277-279 

Attraction of magnet 154, 155 

B 
Batteries 218-239 

C 

Cables, submarine 54 to 56 

telephone 54 

Catechism 427 

Cathode 276, 277-27?) 

Circuits 210-282-285-290 

divided - 308 

parallel 282 

series 282-284 

short 282 

Circuit breakers 34 2_ 355 

Compass 157, 158 

declination of 156, 157 

433 



434 INDEX 

Condensers ,.....».;»» » 48 to 59 

action of ■. 48 

capacity of ..\cj, 5^54* 360 

dielectric of . . > . t .48, 50-52 

how constructed 51, 52 

use of 54 to 59 

wiring of 55 to 59 

Conductivity 305, 307 

Chryolite 277 

Curves 213 

drawing 215 

D 
Direct current 356, 358 

E 

Edison meters 300 

Effects of current 298 

Efficiency 341 

Electricity, catechism on 1 1 to 15 

Electrical amalgam 61 to 62 

action of 62 

Electrical machines 60 to 79 

action of 61 

construction of 63 to 79 

f rictional 63 

simple form of 60, 61 

the Holtz 74 

the Toepler 64 to 68 

the Wimshurst 69 to 73 

use of 69 

Electrical wheel 73, 74 

Electrical winds 62, 63 



INDEX 



435 



Electrical work, power 335~355 

Electrical pressure 285, 289-301-318 

Electro magnet 299 

Electro-magnetic induction 322-328 

Electro-magnetic fields 264 to 273 

strength of 268 

Electro-magnetism 175 to 195 

definition of 175, 176 

Snow rule 179 

Electroplating 278 

Electrodes 279 

Electrolyte 253-276 

Electrolytic corrosion 280, 281 

Electrolysis 276-281 

Electropoin 235 

Electrophorus 17, 18 — 39 to 43 

Electroscope 20, 22 — 32 to 36 



F 

Fahrenheit thermometer 52 

Fields of force 327, 328 

Flux of magnet 188-191 

Force — definition of 335 

Formulae for designing circuits 210, 212 

Fuller battery 234 

G 

Galvanometer 299, 300 

Gravity battery 226-234 

Ground 283 

Ground wire 141-145 



436 INDEX 

H 
Horseshoe magnet ........:......,..... 151, 182, 183 

How to make a magnet 148 to 150 

Hydrometer .231, 232 

I 

Induction coil ;...:.:..;;... 319-321 

Induced electro currents 319 — 324, 328 

Ions 277 

Installation of lightning arresters 139 to 145 

Introduction to static electricity » .... .16 to 22 

L 
Lagging, leading currents ..*.»....**.».»..».«*.. 361 

Law of magnetic circuits 202-212 

Leyden jar 18 to 20—43 t0 47 

discharger for 20 

uses of 74 to 76 

Lightning 16 to 23 — 80 to 93 

arresters 16 

brush discharge of 81 

cause of 81 

effects of 80 to 83 

influence of on electric circuits 84 to 86 

protection from 89, 90 

rods, best form of 91 to 93 

surge 85 to 88 

thunder storms 81 to 83 

treatment of person struck by 89, 90 

Lightning arresters 96 to 146 

auxiliary apparatus 138 to 146 

classes of 101 to 103 

construction of 96 to 98 



INDEX 437 

Lightning arresters — Continued. 

diagrams of connections 112, 144, 145 

for A. C. lines — low voltage 106 to 115 

for A. C. or D. C. lines, high voltage 117 to 146 

for signal lines 115, 116 

for locomotive, or trolley car 116 

for telephone circuits 103, 104 

ground connections 141 to 145 

high voltage choke coil 138 

horn shape 133 to 137 

installation of 139 to 145 

low voltage choke coil 138 

oldest form of 96 

parts of 114, 115 

series resistance 102 

shunt resistance 103 

single and multigap . 101, 121 to 127 

spark gap, type of 96 to 98 

with magnetic blowout 105 

2,500 volt A. C. type 118 

3,500 volt A. C. type 119 

10,000 volt arrangement 120, 121 

Line 283 

Lines 85 to 95 

cutting in, or out 88 

dead, and live 85 to 87 

of force 319-324, 328 

protection of from lightning 93, 95 

M 

Magnet, action of current on 170, 

attraction of 154, 155 

care of 150 



438 INDEX 

Magnet — Continued. 

electro 179 to 183 

flux of 188 to 191 

horseshoe 151, 182, 183 

how to make 148 to 150 

lifting power of 182 to 187 

natural 147 

permanent 174 

Poles of 153, 154 

result of breaking 162 

strength of 182, 184-188 

Magnetic circuits 198 to 201 

equator 159 

field 168, 169-176 

force 165, 166 

induction 169, 170 

lines of force 167, 264, 266 

needle 152 

inclination of 157, 158 

influence of electric current on 176 to 179 

north pole 155-156 

permanence 196 

permeability of metals 161 

reluctance 196 

reluctivity 196 

retentivity 197 

traction 274, 275 

Magnetism 147 to 167 

electro I 75" I 9S 

induced 170 

Magnetizing force 202, 203 

Mass and weight 336 

Megohm 301 



INDEX 439 

Meter bridge 331, 332 

Michrom 301 

Mil, definition of 82, 304 



N 

Natural magnet 147 

North pole 155, 156 



O 

Ohm 301 

Ohm's law 292, 297 — 311, 318 

formulae for calculations 292, 295-312 

problems in 3 I 3"3 I 8 

Oldest form of lightning arrester 96 



P 

Parallel circuit 314 

Permeability of metals 216, 217-273 

Power 337, 338 

Power factor 362 

Potentiometer 333, 334 

Primary batteries 218 

action of 223 to 226 

carbon cylinder for 223, 224 

caustic potash cell 236, 237 

cell materials 220, 221 

classification of cells 222 

construction of 219 

dry cell 238 to 240 



440 INDEX 

Primary batteries — Continued. 

Edison cell 235 

Fuller cell 234 

gravity cell 226 to 234 

names of parts 221 

simple form of 219 

use of in railroad service 223 to 226 

wasting of 239 

R 

Residual magnetism 197 

Resistance 284, 285 — 302, 310 

calculations of 309, 310 

laws of 302 

Rotary converters 365-407 

A. C. side of 400 

Allis-Chalmers type 384-388 

causes of sparking 381 

construction of 369 

erection and care of 372, 375 

instructions for operating 401, 405 

inverted type 405-407 

location of 371, 372 

oscillator for 382, 384 

parallel operation of 399, 400 

principles of 365, 366 

repairs to 376, 382 

starting 390, 392 

synchronyzing of 392, 394 

types of 365, 366—387 

use of : 365-368— 370 

Westinghouse type 378, 380 — 407 



INDEX 441 



S 



Static electricity 23 to 47 

definition of .. 23 

distribution of JJ, 78 

effect on bodies 28, 29, 34, 37 

how produced 24 to 27 

nature of 28 to 33, 76, Jj 

Self-induction 359 

Solenoid 190 

Storage batteries 242 to 263 

catechism on 250 to 257 

chloride accumulator 245 

chloride cell 245, 246 

charging 242, 254, 255 

commercial type 244 to 248 

depreciation of 256 

diagram of connections 263 

disintegration of plates - 2 59> 2 66 

Edison type .248 

for signal work - 254 

in lead lined tank 255 

in railway service 264 

lead grid for 243, 244 

lead zinc type 247, 248 

normal rate of ... . 252 

overcharging of 255 

purity of electrolyte 262 

shortcircuiting of •. 260 

simple form of 242, 243 

sulphating 257, 258 

troubles and remedies 257 to 262 

warping of plate 259 



ELECTRIC RAILWAY TROUBLES 



AND 



HOW TO FIND THEM 

By PAUL E. LOWE, M. E. 



THIS is the latest and most 
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ELECTRIC RAILWAY 
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OPERATORS' WIRELESS TELEGRAPH 
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[P-TO-DATE and most com- 
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Practical Telephone Testing 

By W. G. and D. R. MIDDLETON 



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^PRACTICAL 

TELEPHONE 

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